After winning a gold and a silver medal in her first attendance at a competition at the 1990 Commonwealth Games, O'Neill never failed to win a medal at any international meet she attended, right up until her final Olympics in front of a home crowd in Australia. At the 2000 Australia Trials before the Sydney Olympics, she broke the 19-year-old world record of another "Madame Butterfly", Mary T. Meagher, in the 200m butterfly, but was beaten at the Sydney Games by American Misty Hyman, in an upset.
O'Neill born on 2 August 1973 in Mackay, Queensland. Her family moved to Brisbane and was she educated at Lourdes Hill College in Hawthorne. Whilst at Lourdes O'Neill excelled in sport, setting school records in 50 m and 100 m butterfly, freestyle, and backstroke. She was also LHC cross country champion and set records for the 13 years 800 m in 1986 and for the 15 years 400 m in 1988 for athletics. All these records still stood as of 2011.
O'Neill is an ambassador for the Fred Hollows Foundation. She is also ambassador for companies such as SAAB and Kellogg's, and has her own line of swimsuits that is sold in Target stores throughout Australia.
On 14 July 2000, O'Neill was awarded the Australian Sports Medal for "her significant contribution as a competitor in swimming". On 1 January 2001, O'Neill was awarded the Centenary Medal for "For service to the community through health". At the 1997 Australian Day Honours, O'Neill was awarded the Order of Australia Medal "for service to sport as a gold medallist at the Atlanta Olympic Games, 1996."