 As a youth, O'Sullivan played in the 1988 Quebec International Pee-Wee Hockey Tournament with The Boston Junior Bruins minor ice hockey team.A top prospect out of high school, O'Sullivan was drafted by the Calgary Flames in the 2nd round, 30th overall in the 1992 NHL Entry Draft.After being drafted O'Sullivan chose to play in the college ranks and played with the Boston University Terriers for 4 years.