O'Callaghan was selected in Ireland's 24-man squad for the 2012 Six Nations Championship. He started against Wales, Italy, France, Scotland and England. O'Callaghan featured in all of Ireland's tests in the 2012 tour of New Zealand, coming off the bench in the first, second and third tests. He also featured in the 2012 Autumn Tests against South Africa and Argentina, earning his 90th cap for Ireland in the Argentina test.