Her crowning achievement was a gold medal in the 5000 m at the 1995 World Athletics Championships. She won silver medals in the 5000 m at the 2000 Olympic Games and in the 1500 m at the 1993 World Championships. She has also won three European Championship gold medals and two World Cross-Country Championship gold medals.
On 14 February 2007, the multi-medallist said she planned to hang up her spikes after the Great Ireland Run on 15 April, having said the previous year that she intended to run competitively until the 2012 Olympic Games in London. O’Sullivan later dismissed the rumours that she is retiring after the BUPA Great Ireland Run, adding that she would be retiring at the end of the 2007 season.
In March 2008, O’Sullivan was appointed team manager of the Australian team for the 2008 IAAF World Cross Country Championships in Edinburgh, Scotland.
O’Sullivan ran in the Cork City Marathon, she is expected to run in the Dublin Women’s Mini-Marathon and the Dublin Marathon.
She is known for her dramatic kick at the end of races, clocking 28 second final 200m splits in some of her races.