O'Neill grew up in the community of King City, north of Toronto, playing minor hockey for the King City Kings MHA as a youth. He grew up with two older brothers, Don and Ryan, both of whom played hockey. O'Neill played in the 1990 Quebec International Pee-Wee Hockey Tournament with the Richmond Hill-Vaughan Kings minor ice hockey team from the Ontario Minor Hockey Association. As a 14-year-old, he played at the Midget level with his brother Ryan — almost three years younger than some of his peers. At 15, O'Neill signed with the Thornhill Thunderbirds Jr. A club, whose coach Scott McLennan stated he had the skills to make it to the Ontario Hockey League. O’Neill’s number was later retired by the Thunderbirds in 2004. He finished his rookie season second in the Metro Junior Hockey League in scoring. O'Neill was the first overall selection of the Guelph Storm in the 1992 OHL Priority Selection, held at Maple Leaf Gardens in June 1992. He was named OHL Rookie of the Year that year after scoring 79 points in 65 games with the upstart Storm. His point total of 79 for a 16-year-old was the second highest total by a 16-year-old since Kirk Muller recorded 112 with the Guelph Platers in 1982–83. He also played in the American Hockey League for the Springfield Falcons. O'Neill spent three years in the OHL with the Storm before jumping to the NHL with the Hartford Whalers in 1995. Also in 1995 he represented Canada at the world junior hockey championships winning a gold medal.
NHL
O'Neill was drafted in the first round, fifth overall by the Hartford Whalers in the 1994 NHL Entry Draft. He went to the 2002 Stanley Cup Final with the Hurricanes before losing to the Detroit Red Wings. O'Neill was selected to the Eastern Conference team for the 2002–03 NHL All-Star Game. On July 30, 2005, the Hurricanes traded O'Neill to the Toronto Maple Leafs for a conditional draft pick in the 2006 NHL Entry Draft. He expressed a desire to play for the Maple Leafs, his hometown team, before the signing of the CBA. Re-united with former Carolina Hurricanes coach Paul Maurice, O'Neill enjoyed a resurgence early in the 2006-07 season and, benefiting from playing on the top line with Mats Sundin and Alexander Steen, was briefly one of the top scorers. However, towards the end of the season, O'Neill had been benched for the remaining games due to poor offensive performance. His poor performance combined with his fear of flying led him to consider retirement at the end of the season. He was invited to the Carolina Hurricanes 2008-09 Training Camp, but did not make the final roster and was released.
