 He first worked for Wilford and Levi, a law firm, then went into partnership with a university friend Frank Kelly as O'Leary and Kelly.In 1919 he was invited to join the Wellington legal firm that would become known as Bell Gully.O'Leary was President of the Wellington Law Society 1921-22, and the New Zealand Law Society 1935-46, and on the Victoria University College Council and the University of New Zealand Senate.