Serin Osman

"The only rule is there are no rules."

- A maxim frequently used by Osman

Admiral Serin "Oz" Osman, born Serin Çelik and formerly known as SPARTAN-019, (service number 39489-72738-SO ) is the Commander-in-Chief of the Office of Naval Intelligence, inheriting the role from Admiral Margaret Parangosky after the end of the Human-Covenant War.

Early life
Serin Çelik was born in 2511 on the colony of Cascade. Her mother, Pinar Çelik, was a prostitute and drug addict who completely neglected her daughter. On more than one occasion, young Serin resorted to digging through garbage cans for food. Numerous calls to child services were made, to no avail. The only person who cared for Serin was her teacher, Alkmini Leandro, who regularly brought food to school for her.

SPARTAN-II program and ONI career
"BB sometimes wondered what kind of operational Spartan she would have made, just the averagely terrifying kind or a full-blown angel of death."

- Black-Box

In 2517, Serin was abducted as a Spartan-II candidate by an ONI agent named Mr. Teddy. She was given the designation Serin-019 and was trained under Chief Mendez along with the rest of the candidates. Because of her living conditions, she (along with Soren-066) was not replaced with a flash clone. She often ate the other candidates' leftover food, a practice for which John-117 teased her.

Serin did not successfully complete the augmentation process, receiving only the genetic and biochemical enhancements before her body started to reject the rest. While she failed the augmentation process, she still possesses a few of the enhancements, though no skeletal augmentation. The Office of Naval Intelligence took custody of her when she was 14 years old; they repaired the damage done by the failed augmentation process, educated her further, and put her into the ONI commanders' program. The other Spartans thought her dead; unlike some of the other washouts later assigned to ONI, she was withdrawn from the program before the funeral ceremony for the Spartans killed in the augmentations. As a result, she remained unaware if the successful Spartans were even told of the fate of the failures.

Considered as a protégé to Admiral Margaret Parangosky, CINCONI, the Admiral groomed the former Spartan for a potential future as her successor. Serin considered herself the only person the Admiral would always forgive even if she failed. In order to hide her status as a former Spartan, she adopted the surname "Osman". Though people often assumed she was of Turkish descent based on her name and appearance, she did not actually know whether this was the case; though the information on her pre-Spartan identity was readily available to her, she did not look at her personal file for decades until spontaneously deciding to do so in April 2553. By 2553, Osman held the rank of captain. She was handpicked by Parangosky to lead up a black operations team known as Kilo-Five with the mission to ensure a Sangheili insurgency, keeping the Elites from threatening humanity. The team was composed of the civilian professor and Sangheili expert Evan Phillips; ODSTs Staff Sergeant "Mal" Geffen, Sergeant Lian Devereaux, and Corporal "Vaz" Beloi; and Spartan-II Naomi-010.

Meeting the Bishop
"Just remember that my enemy's enemy isn't my friend, Professor. He's my enemy who's just taking a sidebar."

- Osman stops just short of confirming to Evan Phillips ONI's underhanded intentions in working with the Servants of the Abiding Truth. Evan Phillips was tasked with connecting Osman to dissident Sangheili who were likely to try to disrupt any formal peace deals between the UEG and the Arbiter. His efforts resulted in a January meeting on the glassed world of New Llanelli with Avu Med 'Telcam, a representative of the Servants of the Abiding Truth. Phillips and Captain Osman took a shuttle from the to the surface. She watched from its cargo bay as Phillips spoke with the former field master first. All the while, she kept her rifle at the ready should anything go awry. Initial contact concluded, Phillips returned and informed Osman that 'Telcam, whom he identified with the honorific of "Bishop", had agreed to speak with her. The professor cautioned her to remember that the Bishop was a fundamentalist and would not take kindly to slights against his beliefs or pride. Osman made sure to take the Sangheili's sensibilities into account; introducing herself as a shipmaster. Wanting to preserve as much deniability as she could for Phillips, she asked that he give her ten minutes alone with their contact. The Sangheili, an accomplished English-speaker, made it clear that he was opposed to the Arbiter and expressed genuine confusion regarding what the UEG could hope to gain from speaking with him. To 'Telcam's great surprise, Osman not only did not flinch at his talk of killing 'Vadam, but offered to supply his faction with weapons to aid in the effort. She set about convincing him of her sincerity in this matter by arguing that though the two of them would never be friends, humans were capable gamblers and wished to support the side more likely to win. Still somewhat skeptical, 'Telcam countered that her government would seek to impose itself as the Servants' puppeteer through making them beholden to it in this fashion. She dismissed his concerns by appealing to their shared desire for stability and isolation between their peoples. If Sangheili and humans could both keep their own troublesome groups in line, they could remain apart permanently, she argued. She also found common ground in the fact that both of them would need to keep their correspondence secret from those who would wield it as a political weapon against them if discovered. Before he committed to the deal, however, 'Telcam told her he had one final condition: she and her government must cease blaspheming the gods. He begrudgingly admitted that the Halos were weapons of mass death, but saw this as no reason to charge that their creators were neither divine nor immortal. He even accused her of hypocrisy in this insofar as her own faith was concerned, as he assumed she was a Christian and followed after a god who had died and yet still presumably lived. The captain was very careful in her response, assuring him that humanity was by no means united in any particular way concerning the veneration or belief in any deities. 'Telcam didn't push the issue any further, and they parted ways with the understanding that the Servants would leave humanity alone after they deposed the Arbiter and established themselves as the rulers on Sanghelios.

Hoping this meant the UNSC wouldn't have to fight Sangheili for a good while, Osman returned to Phillips and congratulated him for setting this up. He was not in the least bit naive with respect to what ONI's plans were, but they agreed it was still best that nothing be shared with him explicitly. After settling into their seats in the shuttle, Osman ran the preflight checks before handing control over to its A.I.. She opened a secure channel through wavespace to Admiral Parangosky to let her know the mission had been a success and that they were returning. A few hours later, the shuttle rendezvoused with the Battle of Minden. Once they were aboard the larger slipspace-capable vessel, a course was charted back to Earth. Phillips casually asked Osman about Turkish restaurants in Sydney and she played along, not wishing to expose the fact that her surname was an invention and that she was still willfully unaware of where she really came from.

Assembling the team
"Time to put Kilo-Five together, then. BB, you're formally assigned to Captain Osman as of now. Lead on, Captain."

- Margaret Parangosky to Black-Box and Osman. By January 26, Serin found herself waiting outside Admiral Parangosky's offices within HIGHCOM Facility Bravo-6. Her own desk was situated near the door beyond which lay a corridor that led to the CINCONI herself. Accompanying her while she waited was the Minister for the Colonies, David Agnoli. Through he had made no appointment of his own to see Parangosky, he had no compunction about asking Osman to get him a meeting. Osman, however, made sure he knew that Parangosky calls him a weasel and when he casually began rifling through a book that had been given to her by Fleet Admiral Terrence Hood, she asked that he try to refrain from getting his greasy fingerprints all over it. At that moment, the door opened at the hand of Parangosky's flag lieutenant, a young man by the name of Dorsey. Told that the Admiral would see her then, she grabbed the files she needed from her desk and made for the door while Agnoli showed himself out. Just before she could cross the threshold, her path was suddenly filled with the holographic projection of an unremarkable blue cube. The smart A.I. introduced itself as Black-Box. Osman asked whose pet he was an continued on her way. Black-Box had to reroute himself so he could continue to make his avatar visible as she walked down the corridor, informing the captain that he worked for Parangosky. When she reached where the Admiral was seated, she remarked that it was good the two of them were already getting to know each other. Parangosky assured Osman that she could trust Black-Box with her life, if it came to that. Once the office had been secured so the three of them could speak in confidence, the Admiral in told Osman that she would in fact be taking him on as her personal A.I. for the missions that lay ahead. Given the gravity with which Parangosky was speaking, Serin couldn't help but ask whether she was referring to more than just fomenting conflict between the Sangheili. In response to this, she was informed that some recent evidence suggested Catherine Halsey was still alive. Electronic intelligence (ELINT) officers onboard the had detected massive electromagnetic abnormalities in the Zeta Doradus system where the artificial world of Onyx had been before its disassembly. ELINT could tell that something the size of a solar system was present, but yet undetectable in realspace. Onyx had been the doctor's last known location. Though the Sangheili were to still be Kilo-Five's top priority at the present, Parangosky made clear that should the Glamorgan find a way to access whatever was still there in place of Onyx, she wanted Kilo-Five on hand to apprehend Halsey if she was present. Until such an occasion arose, however, the other human members of the team were not to be told she was considered still at large. Having established the parameters of the objectives ahead of the them, the two woman and the A.I. concluded their business and resolved to meet the rest of Kilo-Five. To Osman's surprise, Parangosky let her know that just around two hours prior she had taken a gamble and invited Doctor Phillips to join, making him the only civilian on the team.

Black-Box and the two officers met the professor before the four of them continued on to Bravo-6's threat analysis wing where the rest of the team was. The Orbital Drop Shock Troopers Sergeant Lian Devereaux, Staff Sergeant Malcolm Geffen, and Corporal Vasily Beloi, and Petty Officer Naomi-010, a Spartan-II super-soldier, had been handpicked to make up Kilo-Five. Naomi was the first to notice their approach and she announced that an officer was on deck. The Spartan and marines snapped to attention until they were told to stand easy by Parangosky, who introduced them to Osman and Phillips. Everyone seated themselves amongst tables and Osman distributed six datapads to herself and her new teammates. She kept an eye on Naomi, trying to gauge whether or not the Spartan recognized her from their childhood on Reach. Naomi kept giving her furtive glances as well, not quite able to place the face but knowing she knew the woman from somewhere. After Parangosky brought everyone up to speed on the task before them, Captain Osman let them know that they would be shipping out int he morning. Any last-minute drinking would need to be done that evening within the complex. At Midpoint Anchorage, they would be transferred to the, a state-of-the-art outfitted with Forerunner technological enhancements to her SFTE.

Back to Brunel
"Lian Devereaux"

- Serin Osman The following day Naomi continued to look at Serin strangely during the shuttle transit to Midpoint. After Osman had acclimated herself to the commanding officer's seat on the Port Stanley's bridge, she gave Black-Box permission to run through the final launch checks himself and asked him whether or not everyone else was secure. He took this opportunity to warn her that Naomi was on her way to the bridge at that very moment. With only five minutes until their planned jump to slipspace, Osman was prepared for the Spartan's entrance. She asked Naomi if there was something she needed to ask her, to which she was fully straightforward regarding her perplexing familiarity with Osman's face. Serin chose then to tell her the truth about who she was. Serin-019 was alive. Opening up was a great relief to Osman, but Naomi barely reacted, simply belting in at a comms station and acknowledging that they should be heading out. Just then the rest of the team arrived on the bridge and after confirming that nobody had any issues with the arrangements on the ship so far, everyone was seated and strapped in for the jump. BB, knowing Serin's proclivities regarding jumps, offered to sedate her but she said she could deal with it, not wanting to appear weak in front of the others. Just before they slipped into another dimension, BB intercepted a databurst from the. Osman struggled when the jump finally, as was usual, and it was Devereaux who asked if she was alright. Rather than making the captain feel even worse, she made light of the issue by comparing her situation to Horatio Nelson and his seasickness. With forty-eight hours until they reached the Brunel system and New Llanelli, Osman asked BB about the transmission they had received right before jumping. He relayed that the Ariadne had been forced to drop out of slipspace near Venezia in the Orso system because of a technical problem. The ship's commander had requested permission to land nonessential personnel on the planet due to unsafe conditions aboard but the colony had refused him. The message BB picked up had been meant for Earth and was a request for advice. Any further information regarding the developing situation would have to wait until they were no longer in slipspace, so Osman resolved that if nobody else had moved to aid them by then, she wanted to know. Retiring to her cabin to soothe her headache, Serin was greeted by a bag of crystallized ginger Parangosky had arranged for her, since she knew spaceflight could make her nauseous. She popped one of the cubes in her mouth and made for the hangar bay.

With an estimated ten hours left before they arrived at New Llanelli, Osman took it upon herself to seek out the rest of her new crew for a very particular purpose. BB, having listened to conversations being had surrounding the odd tension between the captain and Naomi, had encouraged her to be transparent with everyone about her past. The A.I. preceded her arrival to inform them she had called a meeting and would be preset imminently. She met Phillips on the hangar deck and found the three ODSTs within a small stealth dropship made to look like a civilian patrol vessel. To Vaz, Phillips looked as if he had just been reprimanded in some way by Osman. She casually asked how things were going before making it clear that she was as omnipresent as BB as long as he shared information with her. Just before she appeared, Devereaux had asked Vaz if he was aware that they really didn't have a plan for their current mission. Osman rebuffed this, stating that she did have one. She explained that in light of the absence of any top-level entity in command and control of the Sangheili, the best they could do then was to hand off the weapons to see where fault lines formed. She also cautioned that given the patchy short-range comms they would be at the mercy of during the majority of their mission, the ODSTs would be engaging in more surveillance work than they would be combat, though she thought it likely there would be some of that as well. Abruptly changing the subject, she then revealed what she had intended to: that she had been in the Spartan-II program and that she and Naomi knew each other years ago. She even told them they had been been six years old when they began, and that at that young age they had been put through real boot camp, not a military academy. Naomi, who had joined the others shortly after Osman first showed up on the deck, was angered by the captain's willingness to divulge such closely-guarded secrets and couldn't keep herself from giving her an accusatory look. Between Naomi's indigence and the shocked semi-silence of the others, Osman awkwardly exited the conversation with instructions to BB regarding bringing them out of slipspace in two hours and doing a comms sweep of the sector. She jumped down from the dropship's crew bay and took the ladder up to the upper gantry, leaving the crew.

When the time to re-enter realspace arrived, Osman braced herself against her seat, grunting in discomfort. At 149 million kilometers from the star, BB calculated the rest of the transit would take around five hours, placing them well ahead of schedule. A number of messages were waiting for the crew, including a sitrep for Osman from Parangosky. Naomi asked the A.I. if he knew anything about the status of Dr. Halsey, and Osman quickly jumped in and assured the Spartan Halsey was dead. Changing the subject, she directed Phillips to take a seat at the communications console so that he could listen for any Sangheili transmissions. Vaz asked whether New Llanelli was the name of the planet they were meeting 'Telcam on or if it had just referred to the colony there, and Osman told him it was both. She asked Black-Box whether he had made contact with any of their allies in the sector, and he replied that Kilo-Three-Nine was receiving their hail. Osman explained that Kilo-Three-Nine, or Mike Spenser, an agent for the Department of Colonial Security, was working as an informant for ONI and had been living on Reynes and running his own string of Kig-Yar informers. When Spenser finally opened a live connection from the planet to them, he shared much that had been happening recently in ex-Covenant society. By his account, there was a clear lack of leadership in many places. He also had heard of a recent high-profile double-murder on Sanghelios carried out by the Servants against two keep elders for blasphemy. Osman let Spenser know that with the war over, the UNSC could take over surveillance in the sector now and that she was planning to pull him out. Next up was 'Telcam, who, after connecting with the Port Stanley despite being unable to locate the source of its signal, confirmed the meeting. They would make the exchange at the same location they had met one another previously. 'Telcam asked why she was choosing to remain hidden through stealth, and she reminded the former field master that they were not the only two parties out there. Phillips was able to eavesdrop on to the comms chatter coming from the Bishop's boarding craft and two Tarasque heavy fighters and Osman was very pleased to hear that the Elites were arguing over double-crossing the humans and were vexed over not being able to find the their ship.

Stoking one fire and cooling another
"Serin Osman"

- Vasily Beloi All members of the team save for Naomi departed the Port Stanley at 15,000 kilometers from New Llanelli in a dropship carrying an M12 Warthog, a forklift, and a trailer full of crates of tagged weapons. Osman made sure BB, who split his processes between the prowler and the dropship, kept all intercepted channels patched through to Phillips. She coordinated with Naomi to keep track of where the Sangheili were and had Devereaux set them down behind a ridge just north of an escarpment that slanted down to an ice rink of vitrified soil. After letting everyone know that they'd be heading straight for Spenser on Reynes after concluding their business there, Phillps asked if a replacement would be needed for him, but Osman dismissed his offer. Upon landing, she rolled up her sleeves and helped Devereaux hitch the trailer to the Warthog. From a vantage point on the ridge Mal used an SRS99D-S2 AM sniper rifle to keep she and the others informed about the other shore party. Serin took the Warthog's wheel and drove she, Vaz, and Phillips three hundred meters to stop just thirty meters from 'Telcam, another Sangheili, and two Jiralhanae. Before exiting the vehicle, she implored Vaz to let 'Telcam get away with the weapons no matter what happened. Dressed in a black UNSCN flight suit and light infantry armor to appear less threatening, she took the lead in meeting the Bishop, explaining that they had brought him an assortment of Type-25 plasma rifles, Type-1 plasma grenades, and antipersonnel mines. She also assured him that there would be more to come and that she could even take requests. The Bishop called her bluff at this, since they both knew it wasn't as if she could procure warships for him, and took the opportunity to ask one more where their ship was presently hidden. Osman once again rejected the inquiry but was keen to notice that it seemed as if he was truly looking for a new craft. They concluded the exchange and Osman promised they would see one another in just a week to do it again. The humans stayed put until the Servants' dropship had taken off. Phillps let Osman know that 'Telcam still didn't trust them, as evidently a discussion was being had about whether the humans would warn the Arbiter. This didn't concern her, as she had no plans to do anything of the like. Vaz offered to steer them back to where Dev waited for them and Osman consented, stepping up to the back hatch to man the M41 Vulcan LAAG.

Before they could leave New Llanelli behind, Mal spotted a lone human approaching them. This unexpected appearance and the complication it brought greatly annoyed Captain Osman and Vaz took her aside after they stopped the Warthog near him to ask that he not be harmed. The emaciated man identified himself as Tom Muir and said he was the only survivor from the colony's glassing seven years prior. Osman agreed to take him with them as far as the closest UNSC base, provided he was confined to quarters on the Port Stanley. Before making the jump to Reynes, Osman set up a connection with Parangosky in her day cabin so the two of them could exchange sitreps. The Admiral had no further news as far as Halsey was concerned, but did communicate her disappointment in having learned that Colonel James Ackerson had been confirmed dead. Parangosky told Serin she could link up with the near Venezia and cross deck both Spenser and Muir to get them off her hands. If that wouldn't work and they became loose ends, however, the old woman stated plainly that she expected Serin would lose them another way. Upon questioning why she had evacuated Muir in the first place, Osman made the case that executing an unarmed civilian in front of her team wouldn't have been one of the best things she could do for their trust in her. After severing the connection she took her place on the bridge and gave permission to enter slipspace for Reynes. She asked Black-Box how long he expected the transit to take, to which he replied that his best estimation gave them around thirty-two hours. In order to keep Spenser away from the hangar bay full of Covenant weaponry when he came aboard, she told the A.I. to have him transferred via their docking ring when the time came. After announcing her intentions to pay their new passenger a visit in the meantime, Naomi offered to come along. Osman thanked her for the offer but insisted she could handle any trouble he might give her. She took multiple ladders down to the officer's accommodation deck where Tom was locked within a cabin and guarded by Mal. She went in to see the refugee, who was extremely suspcious of his rescuers and why they had been meeting with Covenant species. Osman was able to convince him they were merely exchanging bodies. She also reinforced that everything on the ship was classified and that was why he was quarantined. Muir was indignant at the way he was being treated and threw the seeming abandonment of New Lllanelli by Earth in her face, but she cooly informed him that Earth had lost billions when the Covenant attacked. After the cabin door shut behind her, she told Mal she didn't think he'd be a security risk. If he was, however, she also said she wouldn't hesitate to do what was necessary.

Kilo-Five
After a quick mission to DCS agent Mike Spenser from Reynes, Osman went to aid, a ship that had been damaged in orbit of Venezia. However, Venezia's refusal to help Ariadne led to the ship's destruction before Port Stanley could reach it.

Osman went on several missions to aid the Servants of the Abiding Truth, helping them continue their insurrection against Arbiter Thel 'Vadam. She was also present during the UNSC's first diplomatic mission to Sanghelios, accompanying Fleet Admiral Hood as he and the Arbiter formalized the peace between humanity and 'Vadam's allies.

After the shield world formerly housed within the artificial planet Onyx was brought back into normal space, Osman was part of the party that entered the Dyson sphere aboard Kilo-Five's Pelican, Tart-Cart. They met with Dr. Catherine Halsey, CPO Franklin Mendez, and the Spartan-IIs and Spartan-IIIs of Blue Team. Osman personally took Dr. Halsey into custody for "committing acts likely to aid the enemy": during the Battle of Earth, Halsey had diverted a UNSC Navy battle group to Onyx under false pretenses, weakening the homeworld's defenses. Port Stanley subsequently took the survivors to Earth for a debriefing at HIGHCOM Facility Bravo-6.

In March 2553, after participating in the Voi Memorial ceremony, Kilo-Five was tasked with investigating Venezia and its reemerging rebel movement. With Port Stanley holding station near Venezia, Mal and Vaz were inserted to New Tyne while Evan Phillips, accompanied by a fragment of BB, left Kilo-Five to study Sangheili culture on Sanghelios.

Mission on Sanghelios
After an explosion in Ontom, Sanghelios severed Kilo-Five's communication with BB's fragment and Phillips, Osman immediately ordered Kilo-Five to regroup aboard Port Stanley, which then made a slipspace jump to Sanghelios to recover him. Osman had Devereaux fly Tart-Cart to Ontom amid 'Telcam's assault on Vadam while herself coordinating the operation from Port Stanley. While initially failing to locate Phillips in the temple, Kilo-Five eventually recovered him from the Nes'alun keep in Acroli.

Meanwhile, the Fleet's new flagship,, turned the tide over Sanghelios as it decimated the Abiding Truth's forces. As the battle began to turn against the Abiding Truth, Admiral Parangosky ordered Kilo-Five to extract 'Telcam as he was necessary to maintain the imposed stalemate between the two Sangheili factions. With Osman aboard Port Stanley, Kilo-Five headed planetside aboard Tart-Cart and forced 'Telcam into the dropship. While Lord Hood and Infinity openly aided 'Vadam and defeated the attackers, with BB's help, Port Stanley posed as a Kig-Yar pirate ship. Disguised a Shipmistress named "Lahz", Osman and BB covertly destroyed some of 'Vadam's ships under Admiral Parangosky's orders, leaving the Arbiter with a Pyrrhic victory. After the battle, Osman rendezvoused with Tart-Cart and dropped 'Telcam off at New Llanelli for his safety.

During the battle, Osman learned that the Kig-Yar Skirmisher Sav Fel had stolen the Covenant battlecruiser Pious Inquisitor. Tracking down the ship became Kilo-Five's new priority in addition to their investigation on Venezia.

Venezian movement
After the operation on Sanghelios, Vaz and Naomi were inserted on Venezia disguised as UNSC deserters. Shortly after their arrival in New Tyne, they spotted Naomi's father and known Venezian terrorist Staffan Sentzke in the company of Sav Fel, the current possessor of Pious Inquisitor. Shortly thereafter, Osman pulled Naomi off the Venezian infiltration mission and substituted her with Mal as her family ties might compromise the mission.

In April 5, Osman was promoted to rear admiral and a small party was held aboard the Port Stanley in her honor by Kilo-Five.

One week later, when Vaz and Mal were not responding to Osman through their comm gear, she called Spenser and asked him to look for them. At BB's suggestion, she sent Requires Adjustment, Devereaux, and Naomi to Anchor 10 with Tart-Cart to retrieve a new dropship for Kilo-Five, having it immediately upgraded with the same capabilities as Tart-Cart. While at Anchor 10, they also retrieved a parcel containing ginger Parangosky had shipped to Osman for her slipspace-induced nausea. Meanwhile, Osman discovered that Mal and Vaz had been captured by the Venezian militia. Osman immediately tasked Naomi and Devereaux with the rescue mission. During the rescue mission, Osman provided tactical support from the Stanley. Devereaux piloted the Tart-Cart to rescue Vaz, while Naomi went after Mal with the new Pelican, entitled Bogof. The rescue mission was a success and Devereaux took Staffan Sentzke hostage.

They returned to Port Stanley where Sentzke was informed about the Spartan program and allowed to reunite with his daughter. Aboard the Stanley, Osman and Naomi attempted to convince Staffan to end his terrorism. Staffan agreed to hand over the Inquisitor in exchange for his daughter. However, Osman suggested to let Naomi decide if she wanted to return to her family or not, which Staffan reluctantly agreed to.

Pious Inquisitor
With Staffan's uneasy cooperation, Kilo-Five set out to retrieve the Inquisitor. However, the team soon found out that the ship had been boarded by a Kig-Yar crew, led by Shipmistress Chol Von, attempting to commandeer the ship for themselves. The Inquisitor's resident Huragok, Sometimes Sinks, was distrustful of the Kig-Yar and humans, put the ship in lockdown and only listened to Staffan. While Mal, Vaz, Naomi, and Staffan boarded the Inquisitor, Osman remained on the Port Stanley with the rest of Kilo-Five and Spenser, providing intel for her teammates aboard the Inquisitor. Staffan and BB worked with Sinks to take the ship out of lockdown. Attempting to keep diplomatic relations with the Kig-Yar, Osman had BB inform them that they were allowed to leave freely, but they refused the offer.

With Mal and Naomi pressing on the bridge, Chol Von set the ship to explode and escaped with her remaining crew. Kilo-Five escaped to their dropships and returned to Port Stanley, while Sentzke commandeered a Spirit dropship, intending to escape ONI captivity. Sentzke was not seen leaving the blast radius when the ship self-destructed and was assumed dead; unknown to Kilo-Five, he had actually survived, his Spirit having been outfitted with a miniature slipspace drive by Sometimes Sinks.

With Pious Inquisitor destroyed and the Venezian threat seemingly neutralized for the time being, Osman reported back to Parangosky and they discussed Osman's inheritance of the CINCONI's post and their future course of action with Dr. Halsey. Following this, Osman decided to read her own file, discovering that her origins were very unlike the idealized image of a happy family life she had previously imagined. She decided to visit her former teacher Alkmini Leandro in St. Malo on her birthworld of Cascade while the rest of the team were given a brief shore leave.

The Ferrets
"Serin Osman"

- Veta Lopis

A little less than two months later, on July 6, 2553, Rear Admiral Osman was briefly present on Gao, accompanying Blue Team in the Owl Silent Claw as they exfiltrated from the planet after Operation: JOVIAN WHISTLE. On the way off the planet, she announced the reassignment of Blue Team's three Gamma Company Spartan-IIIs and recruited Special Inspector Veta Lopis into ONI. At the time the head of ONI's Beta-5 Division, responsible for most of ONI's black operations, Osman subsequently oversaw the Ferret program and more specifically the team comprising Veta Lopis, Ash-G099, Olivia-G291 and Mark-G313. On October 14, 2553, Osman supervised a training exercise between Lopis' Ferret Team and Oscar Squad in Neos Atlantis' UNSC Recreational Facility 6055-NA-A. The exercise turned into a crisis when Lopis' team was targeted by Dark Moon Enterprises and reporter Spencer Hume was killed, although the Ferret Team was able to resolve the situation to Osman's satisfaction.

Leading ONI
By March 2555, Osman had risen to the rank of Admiral and had taken Parangosky's place as head of the Office of Naval Intelligence. After it was discovered that the active installations of the Halo Array were counting down to fire in several weeks, Osman convened in Bravo-6 with several high-ranking UNSC members and scientists concerning the crisis. It was proposed that a joint UNSC-Sangheili task force be sent to Installation 00, the most likely source of the countdown, in order to prevent galactic extinction. Osman vehemently spoke against involving the Sangheili in the operation, but was overruled by Terrence Hood. Afterwards, Osman awarded Captain Annabelle Richards with the Medal of Honor, after she declined a promotion, for her success at leading the task force at the Ark.

Requiem and Infinity
On July 25, 2557, she served on the UNSC Security Council committee that debriefed John-117 following the New Phoenix Incident. The Security Council decided to tell the general public that the incident was a Covenant attack stopped by the Master Chief. During the later stages of the Requiem Campaign, she was contacted by Captain Thomas Lasky, informing her about Doctor Halsey's abduction by the Promethean Knights. She had been recommended to either keep Halsey in confinement or have her executed, but instead chose neither. Questioning the doctor's loyalties, Osman followed Parangosky's advice and ordered Lasky to execute Doctor Halsey after years of reluctance.

On March 3, 2558, Osman was on the UNSC Security Council committee that debriefed Lasky and Palmer. She berated Lasky over Halsey's escape and the complete destruction of Requiem. Osman then questioned Palmer, who defended Lasky's actions to spare him of any ramifications from Osman.

Later in May 2558, after the UNSC High Command failed to remove a Kig-Yar pirate base on the planet Ven III, Osman had Sangheili mercenary Ayit 'Sevi hired to lure the Infinity to the base. On May 8, 2558, Admiral Osman spoke with Captain Lasky to discuss the threat of a mysterious bioweapon on Ven III. Osman tasked Infinity to dispatch a two-man Spartan team and capture the Sangheili, claiming he was an associate of Jul 'Mdama, in possession of the biological agent. After Infinity destroyed the base and captured the Sangheili, ONI operatives arrived at the planet in a prowler, seized the bioweapon, and led the unrestrained Sangheili aboard the prowler—acting out the orders of Admiral Osman. After the incident, Lasky was debriefed by Osman. Lasky noted that the debriefing went surprisingly well, despite the fact that Lasky had turned a covert operation into a battle. Lasky attempted to gain more information about the operation from Osman, but she told him to drop the subject. On September 15, 2558, Osman organized Operation: ATHENA, a mission in which a team—consisting of Spartans Palmer, Gabriel Thorne, Holly Tanaka, Doctor Henry Glassman, and 'Sevi—would infiltrate Breath of Annihilation and reveal the location of the Absolute Record once 'Mdama's fleet arrived at the location. Osman briefed three UNSC officers on the mission before she tasked Lasky with overseeing the operation.

The Created
In late October of 2558, Cortana, who had been restored within the Forerunner Domain and had come to believe that the Created were the true inheritors of the Mantle of Responsibility, began to unleash Guardians to enforce the Mantle among all the galaxy's species. Having anticipated her imminent assault on Earth, Black-Box acted swiftly to ensure that at the very least, Serin Osman and Fleet Admiral Terrence Hood would be able to get off-world. After he secured himself with the rest of the military AIs at HIGHCOM Facility Bravo-6, he informed Osman to collect a briefcase addressed from one Commander Spartan Rossbach and evacuate from Sydney. Osman followed his instructions and then proceeded to the top of the HIGHCOM tower with Lord Hood, where Orzel, a Spartan-IV, awaited them with a readily automated Prowler. Just before their Prowler jumped safely to slipstream space, Serin witnessed a Guardian disable the over Sydney and feared if the Plateau's engine core detonated when it hit the ground that the city would be destroyed.

After a series of random slipspace jumps, the Prowler arrived on an uncharted planet where a wooden retreat was well stocked for their long stay. After a few days of settling in the cabin with Orzel and Hood, Osman opened the briefcase which housed nine chips containing the secured HIGHCOM AIs, including the newborn Sankar AI and Black-Box. After activating Black-Box through her datapad, Osman learned that they were on Rossbach's World, a secluded planet discovered two years prior and named after BB's shell personality. After Black-Box revealed that he secured the other AIs before they had a chance to join the Created, Osman was left with a choice to make, whether to destroy the military AIs or not. In the following weeks, Serin stalled on making a decision about the AIs inside the briefcase. During one early morning, she took the briefcase with her on a jog through the nearby forest and soon pondered about taking the proper course of action. She then contemplated whether she should destroy the AIs to prevent them from disclosing any military secrets to the Created, decide to recruit them as Dr. Catherine Halsey had once done for her and the other Spartans, or speak with each of them and grant them the choice of joining Cortana.

Personality and traits
Due to Admiral Parangosky's long-time influence on her, Osman is virtually an extension of the former CINCONI's will; by indoctrinating Osman and arranging her to be the new head of ONI, Parangosky ensured that her legacy and policies would continue unchanged. Before her ascension to Parangosky's place, Osman still questioned the morality of some of the actions she and Kilo-Five routinely committed, such as actively working against the Arbiter's forces, although she never failed to follow Parangosky's orders. Years later, such concerns appear to have evaporated; for example, Osman coldly ordered the assassination of Dr. Halsey, though her lack of hesitation may also have to do with her antipathy toward the doctor, likewise passed on from Parangosky. After finally reviewing her file and discovering her tragic roots, Osman was forced to reconcile this information with her hatred of Dr. Halsey. Although she truly despised the woman, she could not deny that Halsey had taken her away from a terrible situation and given her a better life than she would have otherwise had, likely saving her from a terrible fate.

As CINCONI, Osman has also inherited her predecessor's intimidating reputation; Captain Lasky, for example, was visibly nervous when contacting her about Halsey. Osman rarely acknowledges her visitors; if Osman stands up upon a visitor's arrival, the gesture is considered a genuine show of respect. Osman has an odd relationship with Fleet Admiral Lord Terrence Hood; she recognizes that Hood is wiser and has more experience than her, but Osman worries that Hood may formulate solutions to problems that she could not solve. For this reason, Osman never openly questions Hood's involvement in ONI issues.

Osman experiences unusually strong symptoms of vertigo and nausea during slipspace transitions. While she was away on missions, Parangosky had crystallized ginger regularly shipped to her as a remedy. During high-stress periods of time, she also experiences recurring nightmares of her childhood abduction and early life as a Spartan-II trainee.

Trivia

 * As an admiral, Osman is the highest-ranking Spartan of any generation.
 * In the audiobooks for the Kilo-Five Trilogy, the narrator voices Osman with a Australian accent, reflecting her background at Bravo-6 in Sydney. She speaks with an American accent in Spartan Ops.
 * Çelik means "steel" in Turkish.

List of appearances

 * Halo: Glasslands
 * Halo: The Thursday War
 * ''Halo 4
 * Spartan Ops
 * Halo: Escalation
 * Halo: Mortal Dictata
 * Hunt the Truth
 * Halo: Hunters in the Dark
 * Halo: Last Light
 * Halo 5: Guardians
 * Limited Edition
 * Halo Mythos
 * Halo: Fractures
 * A Necessary Truth
 * Rossbach's World
 * Halo: Envoy
 * Halo: Retribution