O'Brien, who grew up in Royton, began acting at a young 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 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 one of very few actors who have 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, although he wouldn't do it with another man. Although the makers of the film insisted it wasn't pornography, his sex scenes with the actress can now be found on some pornographic websites.
To date, O'Brien continues both his television and film acting careers. Recent appearances have been made in The Road to Guantanamo and Totally Frank as well as a reprise of his role in Cracker. Goal! 2: Living the Dream... is currently on general release.
O'Brien was also one of the stars of the BBC's police drama, Holby Blue, appearing from the first ever episode until the third episode of the second series, when his character was shot dead. This makes him the only regular character to have been killed off in the series, not including Neculai Stenga or Mandy French.