O'Brien (Reach)

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

Battle of Reach
He, along with SSgt Avery Johnson, PFC Wallace A. Jenkins, and Private Bisenti, were there 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,.

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 did 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 likely he was the Crazed Marine from the Raid on the weapons cache. When John-117 encounters Johnson's squad, Jenkins and Bisenti are also part of it. The Crazed Marine only mentions Johnson, Bisenti, and Jenkins, and not Mendoza, however Mendoza's body is found later.