[2] His best performance at the World Snooker Championship was reaching the quarter-finals of the 2000 event, where he lost 5–13 to eventual champion Mark Williams.

He retired from professional competition at the end of the 2023–24 season, intending to remain active in the sport as a coach and a commentator for Eurosport.

[3][4] O'Brien is the only player to score a century in their first frame at the World Championships in the Crucible, which he achieved against Alan McManus in 1994 (though he lost the match 10–7 and did not qualify again until 1998).

Victories over John Higgins (in a decider), Barry Hawkins and Ronnie O'Sullivan, before he reached the second ranking-event final of his career by beating Mark Allen by 6 frames to 3.

This run helped him return to the top 32 of the Snooker world rankings 2007/2008, but the early part of the 2008–09 season proved disappointing, O'Brien not reaching the last sixteen of any of the first four tournaments.