He was named in Ireland's training squad for the 2013 Six Nations Championship on 17 January 2013, and came off the bench against Wales and England. An injury to Mike McCarthy meant that O'Callaghan was promoted to the starting XV for Ireland's game against Scotland on 24 February 2013. McCarthy's return from injury relegated O'Callaghan to another bench appearance in the 13–13 draw with France. O'Callaghan was then dropped from the international side, with Leinster's Devin Toner being preferred for the place on the bench against Italy.