Born in Cobh, County Cork, O'Sullivan was educated in Cobh Community College. She completed her leaving certificate in 1987. O'Sullivan's first major international competition was the 1990 European Championships at Split, where she finished 11th in the 3000 m. However, O'Sullivan improved quickly after that, and on 26 January the following year, at Boston, she set a new world indoor record in the 5000 m of 15:17.28, taking more than 5 seconds off the old record. At the time, O'Sullivan (who attained a sporting scholarship) was studying accountancy at the Villanova University in the US, and at the World Student Games at Sheffield in July 1991, she won the 1500 m Gold Medal in 4:12.14, and the silver medal in the 3000 m in 8:56.55. At the start of 1992, she finished 7th in the World Cross Country Championships.