
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.

9 Songs
2004 · as Matt

Goal!
2005 · as Hughie McGowan

Holmes & Watson
2018 · as PC Smalls

Goal II: Living the Dream
2007 · as Hughie McGowan

24 Hour Party People
2002 · as Nathan

A Cock and Bull Story
2005 · as Gary

The Banker
2004 · as Vain Man

Heroes and Demons
2012 · as (segment "The Banker")

The Look of Love
2013 · as Jimmy Humphries

Spike Island
2012 · as Poster Vendor

Virtual Sexuality
1999 · as Alex Thorne

Cracker: Nine Eleven
2006 · as Mark Fitzgerald

Peterloo
2018 · as Peter Wilkes, a Farrier

The Shock Doctrine
2009 · as Narrator (Voice)

Monster Maker
1989 · as Matt Banting

The Following Year
2021 · as Ben

The Watchman
2016 · as Lee Simpson

Bellman and True
1987 · as The Boy