Battle of ReachEdit
- Main article: Fall of Reach
O'Brien, along with Staff Sergeant Avery Johnson, PFC Wallace Jenkins, and Private Bisenti, were fighting a group of Kig-Yar when MCPO John-117 and PO2 Linda-058 boarded the station to destroy the navigational database of the ONI ship, UNSC Circumference.
Battle of Installation 04Edit
- Main article: Battle of Installation 04
- “Glad you could make it, Chief... we saved some party favors just for you.”— O'Brien greets John-117 on board the Pillar of Autumn
He was picked up with Johnson and the Chief when they took a pelican and flew back to the Pillar of Autumn. When the Pillar of Autumn was boarded, John-117 recognized him (but could not remember his name) and saved him from a Kig-yar soldier. He was killed either when the Covenant boarded the Pillar of Autumn or during the Battle of Installation 04.