Born in Oldham, Lancashire, O'Brien grew up in nearby Royton,[2] and was educated at the Bishop Henshaw Roman Catholic Memorial High School in Rochdale.
O'Brien began acting at an early age and was the star of BBC TV series Gruey by the time he was 15.
In 1993 he also played the role of Lee Jones in Children's Ward, and became a regular in the detective series Cracker, as the titular character's son.
His role is highly unusual in that he had unsimulated and very graphic sex with his co-star Margo Stilley, including genital fondling, female masturbation with and without a vibrator, penetrative vaginal sex, cunnilingus, footjob, and fellatio.
Recent appearances have been made in The Road to Guantánamo (2006) and Totally Frank (2005–06), as well as a reprise of his role in Cracker.
In 2013, O'Brien toured the UK in a stage production of Simon Beaufoy's 1997 comedy-drama film The Full Monty, in which he plays the "absurdly over-endowed" guy.