O'Brien is divorced from his wife Lynn, whom he married on 9 May 1996. They have a son together called Conor (b. 2000), who made a cameo appearance in an episode of Coronation Street. O'Brien's late father, Eddie was also a fan of the long-running TV programme. O'Brien began dating make-up Artist Jill Farimond in 2008, but it was reported in April 2009 that the couple had amicably split. His father died of a heart attack.
O'Brien joined the cast of Coronation Street as the murderous factory boss, Tony Gordon in September 2007. The actor explained in an interview that he uses sense-memory technique, drawing on "something dark from the past", to emote his powerful performances. Asked about female reaction to his character's on-screen conspiracy to murder love rival Liam Connor, played by pin-up Rob James-Collier, O'Brien joked: "I'm sure most viewers believe that it's just a story." O'Brien won Villain of the Year for the role at the 2009 British Soap Awards.
The Manchester show drew the ire of football fans when, in October 2008, several months after the 2008 UEFA Cup Final riots, in a scripted line, O'Brien's evil character made a jibe about Rangers F.C.. Complaints led to a similar line being cut from the script of a later episode. Professing surprise at the outcry, the star said: "I'm a Celtic supporter but I need to be clear that I certainly wasn't going out of my way to antagonise any Rangers supporters."
O'Brien was judged Scotland's eleventh most eligible man in February 2009 by a Scotland on Sunday panel. He once said: "I actually find the heartthrob thing laughable," adding: "I am just a jobbing actor from Scotland and I don't think of myself as particularly good looking."
