O'Brien grew up in Royton, in the Metropolitan Borough of Oldham, in Greater Manchester, and was educated at the Bishop Henshaw Roman Catholic Memorial High School in Rochdale, also in Greater Manchester.
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'.
He is featured in the 2002 24 Hour Party People.
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, footjob 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.
O'Brien strongly defended the film during the controversy that followed, saying that he saw no problem with having sex for a film. He said:
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.
- Whitehouse, Jamie (26 May 2004). "Royton actor defends role in explicit movie". oldhamadvertiser.co.uk. Archived from the original on 8 December 2008. Retrieved 4 July 2008.
- Lawrence, Janie (13 January 2002). "Me, cool and calm?; Nicola Stephenson says she's so clumsy she nearly killed someone while filming Clocking Off. She talks to Janie Lawrence about that, her ideal men (yes men) and life after that Brookside lesbian snog". Sunday Mirror. London, England. Retrieved 15 April 2014.
- Davies, Hugh (18 May 2004). "People who say this sex film is offensive are liars". The Daily Telegraph. London.
- Cavendish, Dominic (19 February 2013). "The Full Monty, Sheffield Lyceum / Bull, Sheffield Crucible, review". The Daily Telegraph. London. Retrieved 27 February 2013.