Born in Coleraine in Northern Ireland, O'Neill began his career in England at Nottingham Forest under Brian Clough.
He emerged as an integral part of the Forest team during the club's European glory days, playing on the right side of midfield as well as in his preferred central role.
O'Neill won 64 international caps for Northern Ireland and played for his country in the 1982 World Cup finals in Spain.
During his club career he also played for Manchester City, Norwich and Notts County.
On retirement he embarked upon a managerial career in the lower leagues before his success at Wycombe Wanderers brought him into the spotlight.
As 1993 Conference champions, Wycombe earned a place in the Football League and followed it up with a second successive promotion into Division Two.
In 1995 he moved on to Norwich but resigned just six months later following clashes with chairman Robert Chase over funds for team building.
He was appointed as the new manager of Leicester just weeks later, guiding them into the Premiership via the play-offs at the first attempt, before winning the League Cup the following season.
O'Neill's loyalty has earned him great respect, but the offer to take over at Celtic in the summer of 2000 proved too good to resist.
His move north of the border has seen Celtic re-establish themselves as Scotland's top club, with two successive Scottish Premier League titles.