O'Brien (Reach)

Private O'Brien was one of the UNSC Marines aboard Reach Station Gamma, Dock 9, above Reach.

Battle of Reach
O'Brien, along with Staff Sergeant Avery Johnson, PFC Wallace A. 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 logs of the ONI ship,.

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. 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.

Speculation
It is somewhat likely that the AWOL Marine from the raid on the weapons cache was actually O'Brien. When John-117 encounters Johnson's squad, Jenkins and Bisenti are also part of it. The AWOL Marine only mentions Johnson, Bisenti, and Mendoza, but not Jenkins.