Hugh O'Hagan competed in the men's light-heavyweight boxing at the 1948 London Olympic Games, winning his opening bout against Hans Schwarzmann of Switzerland on points before losing the quarter-final to Adrian Holmes of Australia. In the quarter-final, O'Hagan floored his opponent for a count of nine in the second round before being outpointed in the third. He won his first Irish light-heavyweight title in 1945 boxing out of the Short and Harland club in Belfast, and won the national title again in 1948 while a member of the Corinthians Club in Dublin - a club associated with the Gallagher brothers, building contractors who were known to provide employment for talented boxers from outside Dublin who joined the club.