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 (OHA), 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.