From Halopedia, the Halo wiki
O'Sullivan was a member of the UNSC Armed Forces. Likely a naval crewman, he served aboard the UNSC Spirit of Fire in 2531, when the ship disappeared uinder mysterious circumstances. In 2559, the Spirit of Fire arrived at the extragalactic Installation 00, and proceeded to conflict with the Banished there.
On May 1, 2559, O'Sullivan was one of the crew aboard the Spirit of Fire to notice a Banished operation led by Colony on the surface of the megastructure. He noticed the presence of Banished Huragok, and alerted Captain James Cutter to the situation, who promptly ordered the deployment of Sunray 1-1 and Operation: SPEARBREAKER.
List of appearances