An injury to Peter Stringer early in the 2005/2006 season meant O'Leary had a chance to play, making his Heineken Cup debut against Sale Sharks in October 2005.

O'Leary played a prominent role in Munster's Heineken Cup winning campaigns in 2005/06 and 2007/08, especially in the knockout stages of the 2007–08 tournament.

[8] O'Leary came off the bench in Munster's opening pool game of the 2015–16 European Rugby Champions Cup against Treviso on 14 November 2015.

[10] In October 2016, O'Leary joined French Top 14 side Montpellier Hérault Rugby as a medical joker replacement for the injured Benoît Paillaugue.

O'Leary was awarded Man of the Match for his outstanding performance during Ireland's 27–12 win over Wales at Croke Park on 13 March 2010.

[12] O'Leary recovered to fitness and featured against Italy and France during the 2011 Six Nations, but a troublesome back and a freak eye injury sustained during training ruled him out of the remainder of the tournament.

Injury and the form of Conor Murray kept him out of the Munster team for the rest of the 2010/11 season, but O'Leary was selected in Ireland's training squad for the 2011 Rugby World Cup warm-ups in August.

[14] However, a knee injury suffered by Conor Murray meant that O'Leary was called into the Ireland squad for the remaining 2012 Six Nations games against Scotland and England.