Born on November 28, 1969 in Cobh, County Cork, O’Sullivan is educated in Cobh Community College. She completes her leaving certificate in 1987. Her first major international competition is the 1990 European Athletics Championships at Split, Yugoslavia where she finishes 11th in the 3,000 metres.
O’Sullivan first comes to prominence when winning the 1,500 metres at the 1991 Summer Universiade, before going on to finish fourth in the 3,000 metres final at the 1992 Summer Olympics in Barcelona. She then wins a silver medal in the 1,500 metres at the 1993 World Championships and a gold medal in the 5,000 metres at the 1995 World Championships. She is the favourite for the 5,000 metres title at the 1996 Summer Olympics in Atlanta but drops out of the final due to illness. Her 2,000 metres world record of 5:25.36, set in 1994, stands until 2017.
After a disappointing end to her 1997 season, O’Sullivan makes an impressive comeback in 1998. At the World Athletics Cross Country Championships at Marrakesh in March, she enters both the short course (4 km) and long course (8 km) events. On successive days, she wins both events, and her 4 km time of 12:20 is 14 seconds ahead of her nearest rival. She continues this form into the track season, where her performances in the 1,500 metres, 3,000 metres and 5,000 metres are close to those she had produced at her peak in 1994 and 1995.
At the European Athletics Championships in Budapest, the 1,500 metres and 5,000 metres finals, events at which O’Sullivan usually doubles at major championships, are scheduled to be run on the same day, thus denying her the opportunity of competing in both events. Undeterred, she enters the 5,000 metres and 10,000 metres, having never run the latter event before on the track. In the 10,000 metres final, on August 19, she shadows the leaders, and then produces an astonishing 28.1 second final 200 metres to win the gold medal in 31:29.33 in her debut at the distance. Four days later, in the more familiar territory of the 5,000 metres, the pace is set by Romanian Gabriela Szabo, but again, O’Sullivan produces an explosive finishing sprint to defeat Szabo in 15:06.50.
At the IAAF World Cup held the following month in Johannesburg, South Africa, O’Sullivan wins her second major international 5,000 metres competition of the year, again sprinting clear of the opposition following a very slow pace. She concludes her year by winning the Bupa Great North Run.
At the 2000 Summer Olympics in Sydney, O’Sullivan wins a silver medal in the 5,000 metres. She wins silver medals in the 5,000 metres and 10,000 metres at the 2002 European Athletics Championships, and competes at her fourth Olympic Games in 2004. She is known for her dramatic kick, clocking 28-second final 200 metre splits in some of her races.
O’Sullivan is only one of two women (the other being Tirunesh Dibaba) who have won the short and long course World Cross Country title at the same championship (1998 in Marrakesh).