O'Kane began his career with Derry City F.C. He signed for Nottingham Forest in November 1968 and made his League debut on 30 April 1969 against Leeds United F.C. Initially, he succeeded Terry Hennessey at centre back, but in 1971 he moved to right back and stayed there for the rest of his career. His best period was 1973–76,^[why?] on his return from a broken leg, but injuries were never far away and they finally finished him as a player in the 1975–76 season, when he was still only 28.

During his career, and despite being troubled persistently by injuries, O'Kane made 20 international appearances for Northern Ireland.^[2]

O'Kane moved onto the coaching staff at Nottingham Forest after his playing days and was the only man to have survived throughout the Brian Clough years until Joe Kinnear arrived at the club.^{[citation needed]} O'Kane finally left his position at the club in early 2005 after a 36-year spell.