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 two separate TV shows, Coronation Street and Children's Ward. In 1993 he also became a regular in the detective series, Cracker.
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 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 masturbates 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." 
"Pretty much everything Michael wrote, we did. The general consensus of opinion was to push it as far as we could. The only thing I wouldn't do was have sex with a man—Michael suggested it but that was the only thing I wouldn't do." 
- Whitehouse, Jamie (2004-05-26). "Royton actor defends role in explicit movie". oldhamadvertiser.co.uk. Retrieved 2008-07-04.
- Davies, Hugh (2004-05-18). "'People who say this sex film is offensive are liars'". The Daily Telegraph (London).
- "Director defends sex shocker". Daily Mail (London). 2004-05-18.
- Cavendish, Dominic (19 February 2013). "The Full Monty, Sheffield Lyceum / Bull, Sheffield Crucible, review". London: telegraph.co.uk. Retrieved 27 February 2013.
- "I WANT TO SETTLE DOWN & HAVE KIDS BUT I ONLY EVER FANCY BAD BOYS". Sunday People. 27 February 2005. Retrieved 24 January 2013.