- “I don't miss! I never miss!”— Private O'Brien
O'Brien was a member of the UNSC Marine Corps who participated in the Human-Covenant war, and an Orbital Drop Shock Trooper in the 105th Marine Expeditionary Unit. At some time between 2542 and 2552, he was assigned to a squad led by MSgt Cortez, as a part of the Marine contingent aboard the 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, Cal-141, was the primary shooter.
The squad was deployed to the planet using the cover of a meteor shower. Upon landing, O'Brien's SOEIV was sent crashing to a nearby 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 not happy with the loss of his fellow squadmate, Checkman, who had died during atmospheric entry due to malfunction of his drop pod. Along the way, he needled Cal whenever the chance came, showing his contempt for the Spartan despite the fact that without Cal's help, he would have died in his pod earlier.
During the operation, he was attacked by a Brute chieftain wielding a gravity hammer while attempting to kill a Grunt he had alerted. However, before the chieftain's gravity hammer struck O'Brien, Cal-141 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 barely 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.
Cal saved him 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 anymore Covenant patrols. His anger and contempt towards Cal grew a bit hotter when Dutch reminded him that Cal had saved his life twice already and that he should have thanked the SPARTAN.
The group soon made their way to the sniping position, just as the Prophet's dropship was touching down. While the SPARTAN was preparing her sniper rifle to make the shot, the Brute chieftain ambushed the team. Before O'Brien was 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 repeatedly 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 a female. Cal, realizing that she could no longer function as a sniper due to the extent of her injuries, yelled to O'Brien to take the shot. At first he hesitated, but took the sniper rifle and killed the Prophet.
O'Brien and the team remained with Cal until her death. Back on the Kronstadt, 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 (though it's implied that he threw an apple at the marine). He thought deeply about how Cal had repeatedly saved his life.
- Over the course of the mission to assassinate the Prophet, O'Brien was saved by Cal four times.