O'Reilly was a member of the United States Colonial Marine Corps. He was part of a search and rescue mission sent to LV-426 aboard the USS Sephora in 2179 to investigate the disappearance of the USS Sulaco and its crew 17 weeks earlier. O'Reilly went M.I.A. early during the mission after suffering leg injuries.


O'Reilly was part of Rhino 2-1, one of the advanced teams sent aboard the USS Sulaco to investigate the ship and reinforce Rhino 2-3. O'Reilly was one of the several Marines injured in the internal explosion aboard the Sulaco that occurred just as Corporal Winter was attempting to board the ship via the umbilical from the USS Sephora. He suffered a leg injury that incapacitated him. While he was left with Private O'Neal, he later disappeared, leaving his ultimate fate unknown.


O'Reilly was outfitted with standard-issue M3 Pattern Personal Armor for protection; his armor was fitted with a TNR Shoulder Lamp attachment for illumination. He had customized his armor vest with Celtic symbols on the chest plate and a shamrock leaf with an American flag on his left shoulder pauldron. O'Reilly carried several pouches on his waist around the belt and a pouch on his left leg. O'Reilly also carried an M4A3 Service Pistol.


  • O'Reilly's TNR Shoulder Lamp glitches through the floor.
  • His name and Celtic imagery would suggest that O'Reilly is of Irish descent.
  • It is possible that he was taken by Xenomorphs during the standoff in the Sulaco's hangar before Keyes and Winter returned.