From Halopedia, the Halo wiki
Captain Marisa O'Day served as a drill instructor for the second class of the SPARTAN-IV program. She was tasked with training the Spartans to break their bad habits that they had formed over the course of their military careers. In 2554, O'Day was stationed aboard a space station orbiting a dwarf planet. Spartans in one of her training groups included Edward Buck, Kojo Agu, Michael Crespo, Rudolf Schein, and Kovsky.
Unbeknownst to most, Schein was affiliated with the United Rebel Front. After a training exercise, Schein murdered fellow trainee Hideo Wakahisa and then set a series of explosives around the station. When it was discovered that Schein was responsible for the murder, O'Day, Jun-A266, and Musa-096 confronted him in the station's recreational room. Schein threatened to blow up the entire station unless his demands were met. When Schein attempted to detonate the explosives he had placed around the station, Jun charged at him and knocked the trigger out of his hand. Musa managed to turn off the station's artificial gravity, which caused the explosives' switches to falter and prevented all the explosives in the station from detonating. However, Schein still managed to prime a grenade. While O'Day attempted to avoid the explosion, she was caught in its blast and propelled down a hallway. The explosion knocked her unconscious and set her legs on fire. Buck, Agu, and Crespo grabbed her and left to bring her to safety. As they attempted to escape from the breached rec room, Buck and O'Day smashed into a door. Having hit the door at a bad angle and at a high speed, O'Day died instantly.
After Schein was killed by Jun and the situation ended, Musa contacted Buck, Agu, and Crespo told them to leave O'Day's corpse in the room they were taking shelter in. A clean-up crew came by later to remove her body.
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