O'Neill was a three-time NCAA champion on rings, winning in 1987, 1988, and 1989, as well as holding the highest qualifying average record (9.93) in 1987 and 1989. After college, he went on to compete internationally. O'Neill came fourth at the 1992 World Artistic Gymnastics Championships, the highest American male finisher, then became the first American man to win a medal at the world championships in 15 years when he won the silver medal in men's rings at the 1994 World Artistic Gymnastics Championships in Brisbane.