Though a versatile and dependable performer in reserve fixtures and often praised for his 'O’Neill Shuffle'— a trick of wrong-footing opponents with subtle footwork — his first-team opportunities at Leeds were extremely limited. O’Neill made just one league appearance, coming on as a substitute in a 1–1 draw against Newcastle United on 2 March 1974. He also made two European Cup substitute outings: one replacing Paul Reaney on 3 October 1973 and another replacing Frank Gray three weeks later against Hibernian.

Across the seven seasons from 1967 to 1974, O’Neill remained a consistent Central League player and reserve team utility figure, filling various roles across full-back, midfield, and even forward when needed. Despite his adaptability and professionalism, he was largely blocked from regular first-team action by Leeds’ dominance during the Revie era.

That Leeds side in the late 1960s and early 1970s was arguably one of England’s strongest ever, progressing regularly in domestic and European competitions, and at times fielding squads revered across Europe. For a young full-back like O’Neill, breakthrough chances were few in a side packed with international stars.

With none of that outstanding depth available to him, O’Neill accepted a free transfer to Chesterfield in July 1974, following many contemporaries including Trevor Cherry. That move was initiated under manager Joe Shaw and transformed O’Neill’s career, where he went on to become one of Chesterfield’s most durable servants — making over 430 league starts and five substitute appearances, scoring six goals over 12 seasons. He even helped Chesterfield capture the Anglo-Scottish Cup in 1981 and the Fourth Division title in 1984/85, before retiring in 1986.