O'Brien qualified for the men's 4×200 m freestyle relay, as a member of the British team, at the 2004 Summer Olympics in Athens. He finished sixth in the 200 m freestyle from the Olympic trials in Sheffield, posting a relay entry time of 1:51.12. Teaming with Simon Burnett, Gavin Meadows, and Ross Davenport in the final, O'Brien swam a third leg and recorded a split of 1:49.05. He and the rest of the Brits missed the podium by 0.77 seconds behind the Italians, led by Massimiliano Rosolino, finishing fourth in a new national record of 7:12.60.