"People who say they find it offensive are liars. If they say they find it shocking, I don't believe them. It's only sex. To me they were just scenes we were shooting, to be honest, and I was surprised how ordinary and how natural it was. But it's a different thing for a girl than it is for a lad. I didn't fancy her—I felt protective towards her. On set she was the only woman with a crew of four lads. I know how difficult it was for her. You can't get away from the fact she's a young girl."
O'Brien grew up in nearby Royton and was educated at the Bishop Henshaw Roman Catholic Memorial High School in Rochdale. He began acting at an early age and was the star of a BBC TV series Gruey by the time he was 15. He also featured in several other series at the time in one-off or recurring roles. In 1993, he played the role of Craig Lee in Coronation Street and then the role of Lee Jones in Children's Ward. In 1993 he also became a regular in the detective series, Cracker.
O'Brien was in a relationship with Actress Nicola Stephenson for eight years. It ended in 1999.
In 2001 he played the role of Private Allen Vest in HBO's acclaimed series Band of Brothers where he played a prominent part in the episode 'The Last Patrol'.
In 2003 he appeared in the Cooper Temple Clause music video "Promises, Promises" and can be seen saying he got 'laid' in the Millennium Dome in the making of the video.
In 2004, O'Brien appeared in the controversial film 9 Songs. According to The Guardian, 9 Songs was the most sexually explicit mainstream film to date, largely because it includes several scenes of real sexual acts between the two lead actors. 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 and fellatio. During a scene in which Stilley stimulates his penis with her hand after performing fellatio on him, he became the only mainstream British actor who has been shown ejaculating in a mainstream UK-produced feature.
In 2013, O'Brien is touring 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.
