O'Brien (Reach)

Private O'Brien was one of the UNSC Marines aboard Reach Station Gamma, Dock 9, above Reach. He, along with Staff Sergeant Johnson, PFC Jenkins, and Private Bisenti, were there fighting a group of Jackals when John-117 and Linda-058 boarded the station to destroy the logs of the ONI Prowler Circumference. 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, the Chief recognized him (but did not remember his name) and saved him from a Jackal. He was killed either when the Covenant boarded the Pillar of Autumn or during the events on Installation 04. It is likely he was the Crazed Marine from 343 Guilty Spark, as in The Fall of Reach, when John 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.