He has previously played for the Penrith Panthers, the New Zealand Warriors, the Brisbane Broncos and the Sydney Roosters in the NRL.

O'Sullivan made his first grade debut and scored a try in round 18 of the 2018 NRL season for the Sydney Roosters against the Gold Coast Titans.

Otherwise, he spent most of the year playing for Penrith's NSW Cup team, including their 29–22 grand final victory over Canterbury-Bankstown Bulldogs.

[6] His club debut for the Dolphins was in their inaugural round 1 match of the 2023 NRL season starting at halfback as they pulled off a big upset defeating the Sydney Roosters 28–18 at Suncorp Stadium.

In round 23 of the 2024 NRL season, O'Sullivan kicked a two-point field goal in golden point extra-time which won the game for the Dolphins 34-32 against the New Zealand Warriors.