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This article is about the ODST who participated in Operation: GREY VEIL. For another ODST who died and may be the same person, see O'Brien (Mombasa).
O'Brien is a sniper and an Orbital Drop Shock Trooper in the 105th Shock Troops Division who participated in the Human-Covenant War. At some time between 2540 and 2551, he was assigned to a squad led by Master Sergeant Cortez as a part of the Marine contingent aboard the destroyer UNSC Kronstadt.
He and a team of fellow Helljumpers were charged with assassinating a Prophet who was overseeing Covenant logistic operations on Heian. O'Brien was to serve as the backup sniper in the mission. He felt this was an insult to his abilities and was even more shocked to learn that a Spartan-II, Cal-141, was the mission's primary sniper.
The squad was deployed to the planet using the cover of a meteor shower. Upon landing, O'Brien's SOEIV crashed into a cliff and fell into a swamp. The fall damaged the oxygen tank and the pod began to sink.
Cortez and Dutch hurried to save O'Brien but stopped short due to the thick mud in the swamp. The Spartan calmly walked to the pod, and with some effort, lifted the pod and tossed it out of the swamp. She then pried the door loose to let O'Brien exit. The ODST was shocked by the sudden action and was disheartened by the loss of his squadmate Checkman, who had died during atmospheric entry. Along the way, he frequently needled Cal, showing his contempt for the SPARTAN despite the fact that, without Cal's help, he would have drowned in his pod.
During the operation, he was attacked by a Jiralhanae chieftain wielding a gravity hammer while attempting to kill an Unggoy he had alerted. However, before the chieftain's gravity hammer struck O'Brien, Cal caught it in her hand. However, O'Brien was swept aside and rolled down an embankment, losing both his weapons in the fall. Fortunately, O'Brien managed to grab a root that hung precariously over a waterfall. From this position, he watched the Spartan battle the Brute, which she ultimately kicked off the waterfall after gouging out its right eye.
Cal saved O'Brien again when she pulled him away from the waterfall. However, he seemed unappreciative as he yelled at her to put him down, and then snapped at Dutch, who told him to quiet down before alerting more Covenant patrols. His anger and contempt toward Cal grew more intense when Dutch reminded him that Cal had saved his life twice already and that he should have thanked the Spartan.
The group eventually arrived at the sniping position just as the Prophet's dropship was touching down. While the Sprtan was preparing her sniper rifle to make the shot, the Brute chieftain ambushed the team. Before O'Brien could be struck unaware from behind, Cal saved him, taking the blow from the chieftain's gravity hammer.
The ODSTs tackled the chieftain to the ground and Dutch shot the Brute in the face with his SMG. As Cortez removed Cal's helmet to assess the extent of the SPARTAN's injuries, O'Brien was shocked to discover that Cal was female. Cal, realizing that she could no longer function as a sniper due to extent of her injuries, yelled to O'Brien to take the shot. At first he hesitated, but he took up the sniper rifle and killed the Prophet.
O'Brien and the team remained with Cal until her death. Back on the destroyer, O'Brien remained silent, not saying a word even when the rest of the squad angrily scolded Berger for mocking Cal's abilities in battle. He thought deeply about how Cal had continuously saved his life.
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