In his senior year at Catholic Memorial, O'Sullivan helped lead his team to the State title while also earning two varsity letters in baseball at Governor Dummer. In 1991, O'Sullivan suited up for the National Select 16 and 17-year-old team that competed in the U.S. and Japan before heading to Boston University the following year.

In his first year at BU, O'Sullivan missed the majority of the season due to a neck injury before returning to the line-up in 1993-94. The Dorchester, Mass. native was a member of the 1994 U.S. World Junior Championship team before his game took off during his junior year in 1994-95.

O'Sullivan led BU in scoring and to the National Title while earning Hockey East First Team All-Star honors, NCAA East Second Team All-American honors, NCAA Championship All-Tournament Team honors and NCAA Championship Tournament MVP along the way.

After his senior year in 1995-96 with BU, O'Sullivan made his NHL debut with the Flames the following year playing in 27 games while spending the other half of the season with the American Hockey League's Saint John Flames.

For the next three seasons O'Sullivan shared his time with Calgary and Saint John before he was traded to the New York Rangers in the later part of the 1999 season. After the Rangers acquired O'Sullivan, he was assigned to the AHL's Hartford Wol Pack before being released during the off-season.

Prior to the 99-2000 season, O'Sullivan signed as a free agent with the Vancouver Canucks and went to play ten games with the team before being assigned to the Canucks AHL affiliate in Syracuse.

Following the 99-2000 season, O'Sullivan represented the U.S. for the second time in his career at the 2000 World Championships, five years after his first appearance back in 1995.

In 2000-01, O'Sullivan joined the AHL's Cincinnati Mighty Ducks before deciding to play in Europe in 2001-02.