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O'Brien was a UNSC Marine and Orbital Drop Shock Trooper stationed at Earth in 2552. He was probably part of the 7th Orbital Drop Shock Trooper Division.
During The First Battle of Earth, his squad was dispatched from the UNSC In Amber Clad to relieve the ground forces. Their Pelican was shot down over Old Mombasa by a Scarab walker. O'Brien was killed in the crash.
Trivia
- O'Brien is named after the Bungie employee: Bill O'Brien. He worked on the Animations and Cinematics for Halo 2.