 O'Sullivan missed the majority of his freshman year (1992–1993) after suffering a neck injury during a game versus Boston College.O'Sullivan eventually recovered and became one of the top defensive players in the nation.His most successful season was during the 1994–1995 season when he led the Terriers in scoring with 56 points in 40 games and eventually leading the Terriers to a national championship in 1995, in which he was named the Most Outstanding Player.