O'Brien joined the regular cast as loveable cop Jack Holden in Home and Away and played the character between 2005 and 2009. His most notable storyline involved the on-again-off-again romance with Jodi Gordon's Martha Mackenzie.
O'Brien was named the runner-up, behind Comedian Andy Lee in the 2006 Cleo Bachelor of the Year competition. In 2008, O'Brien took part in the third series of It Takes Two, an Australian reality show in which celebrities compete against each other by singing. O'Brien was partnered with pop singer Jade MacRae. They finished in fourth position.
In October 2013, it was announced that he had joined the cast of Neighbours in the guest role of Eric Edwards.
In December 2017, O'Brien was cast as the lead opposite John Jarrett in the Christmas comedy Christmas Down Under, directed by Louis Mandylor of My Big Fat Greek Wedding fame. Christmas Down Under is due for release in 2018.
