O'Reilly was a member of the United States Colonial Marine Corps from the USS Sephora. O'Reilly was one of the Marines caught in an internal explosion aboard the USS Sulaco that occurred just as Corporal Winter was attempting to board the Sulaco from the Sephora via the umbilical. Although he survived the explosion aboard the Sulaco (with injuries) O'Reilly's fate afterwards is unknown but it is likely he was killed by Xenomorphs aboard the Sulaco as he was nowhere to be found when Cpl. Winter and Sgt. Keyes returned to the hangar with the flight recorder.
O'Reilly was one of the Marines sent aboard the USS Sulaco to investigate the ship. O'Reilly was one of the several Marines injured in the internal explosion aboard the Sulaco that occurred just as Cpl. Winter was attempting to board the ship via the umbilical from the USS Sephora. O'Reilly was being helped by Private O'Neal and Paluski when Cpl. Winter boarded the Sulaco.
O'Reilly was among the several Marines who were presumably devoured by Xenomorphs as he could not be found after Cpl. Winter and Sgt. Keyes returned to the hangar with the flight recorder.