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O'Brian 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 Battalion.
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 by a Covenant Scarab Walker over Old Mombasa. When the dropship made a hard landing outside of Hotel Zanzibar, O'Brian was killed in the crash. It is unknown what other engagements he was involved with. His character is not mentioned during the course of Halo 2.
Trivia
- He is a non-interactible character
- O'Brian is named after the Bungie employee Bill O'Brien, who worked on the animations and cinematic for Halo 2.