O'Neill came to South Sydney in 1965 and his aggressive play in the scrums and charging runs close to the rucks caused him to be noticed. His toughness and solidity earned him the nickname "Lurch", and in his debut season O'Neill played for South Sydney in the Grand final against the champion St George team. A tall and strongly framed man, O'Neill was able to develop consistently as he built up his weight from 88 kg or 14 st. in 1965 to 104 kg or over 16 st. by 1970. Between 1967 and 1971 he played in five grand finals for South Sydney, winning all but the 1969 contest against Balmain.
In 1971 the financial problems at South Sydney caused him along with team mate Ray Branighan to leave for Manly until the end of 1974. There O'Neill played in two more premiership sides, and his battle with Cronulla strongman Cliff Watson in the brutal 1973 grand final (won by Manly 10-7) is regarded as one of the toughest conflicts seen in the Australian game.
In 1975, still a major force in representative rugby, O'Neill returned to South Sydney. Paradoxically, though he was superb in representative games, he was disappointing in club rugby and retired during the 1976 season. O'Neill coached South Sydney in 1977 but could win only one of the last eighteen games and he stood down.
In 2004 he was named by Souths in their South Sydney Dream Team,, consisting of 17 players and a coach representing the club from 1908 through to 2004.In February 2008, O'Neill was named in the list of Australia's 100 Greatest Players (1908-2007) which was commissioned by the NRL and ARL to celebrate the code's centenary year in Australia.