During his boxing career, O'Halloran defeated former title contenders Cleveland Williams and Manuel Ramos. He also defeated Danny McAlinden, who won a bronze medal in boxing at the 1966 British Empire and Commonwealth Games in Kingston, Jamaica and later became the British and Commonwealth Heavyweight Champion. O'Halloran's losses included defeats to Joe Roman (twice), Joe Bugner, Ron Lyle, and future heavyweight champions George Foreman and Ken Norton.