O'Neill was born in Liverpool, England, the son of a police officer and became interested in martial arts from an early age.
[1] He first started to train at Judo, but later applied to join the Liverpool Karate Club, lying about his age to gain admission.
O'Neill's competitive fighting career came to an end in 1982, when he seriously damaged the ligaments of his knee in an International match against Italy.
In 2006 the premier US martial arts magazine "Black Belt" published an unordered list of the "deadliest fighters on the planet", excluding deceased fighters and mixed-martial artists, an attempt to subjectively rate the men according to how skilled they were in their prime, with O'Neill ranked No.1 .
He has also appeared on British television in Civvies (1992), Comics (1993), The Governor (1995), The Commander: Windows of the Soul (2007), Giri (2008) and Above Suspicion (2009).