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, playing Mark, the son of central character Robbie Coltrane. In 1999 he made his cinema debut with the film Virtual Sexuality and began a reunited with Cracker director Michael Winterbottom for 24 Hour Party People in 2002 and, in '04, for the controversial 9 Songs, which included scenes of genuine sexual intercourse between O'Brien and his leading lady Margo Stilley. His role had earned him the credit of being the only mainstream British actor who has been shown ejaculating in a mainstream UK-produced feature. Since 9 Songs, O'Brien has made two further films with Winterbottom; A Cock and Bull Story and The Look of Love. Recent regular TV roles include Holby Blue and The Syndicate.