[2] During his career, O'Neill became an eight-time winner of the Australian National Time Trial Championships and an Olympian in the 2000 Sydney Olympic Games.

At the Junior UCI Track Cycling World Championships in 1992, O'Neill won the bronze medal as part of the team pursuit squad.

In 1995, he won the silver medal in the road time trial at the Oceania Cycling Championships whilst nursing a broken pelvis.

[5] On 12 August 2007, at the Tour of Elk Grove, O'Neill provided a sample which tested positive for the appetite suppressant drug phentermine.

[6] On 13 June 2008 the Court of Arbitration for Sport (CAS) ruled that O'Neill bore "no significant negligence" in registering a positive finding for Phentermine, and was given a 15-month suspension, from 12 August 2007 to 12 November 2008.

Race productions include Tour of the Southern Highlands, In 2016, Nathan started OnEdge Sportsales, LLC with his then fiancé.