Born in St Helens, Merseyside, O'Neil was a schoolboy international at both football and rugby league, but abandoned the oval ball in favour of the spherical one.
However, after Tommy Docherty replaced Frank O'Farrell as manager halfway through the 1972–73 season, O'Neil fell out of favour and was allowed to go on loan to Blackpool in March 1973.
He made seven appearances for Blackpool, and, although he returned to Manchester United at the end of the spell, he left the club for Southport in August 1973.
He made 74 appearances in two seasons with Tranmere, scoring 10 goals in the process, and was transferred to Halifax Town in 1980.
O'Neil eventually ended up back at Manchester United, coaching the club's academy teams from Under-9s to Under-16s until 2005.