Here are some facts about the career of O'Neill, who was born on March 1, 1952 in Kilrea, Northern Ireland.
* Scouted as a teenager by Nottingham Forest in 1971, he became a regular fixture of Brian Clough's midfield in a golden era for the club. Along with helping them to domestic honours, O'Neill played in one of their two European Cup final triumphs before moving to Norwich City and Manchester City.
* His international career from 1972 to 1985 totalled 64 caps with Northern Ireland, memorably helping his team reach the second group stage of the World Cup in Spain in 1982.
* His coaching career began in semi-professional football in 1990 with Wycombe Wanderers, who won two consecutive promotions to go from the Conference to the third division and scoop two FA Trophy titles.
* O'Neill joined Leicester City in 1995 after only a few months at Norwich and in a five-year spell brought them promotion to the Premier League and two League Cup titles.
* Departs for another five-year stint, this time at Celtic, and wins the Scottish domestic Treble in his first season. They went on to win two more league titles and two more Scottish Cups, and to reach the 2003 UEFA Cup final, before he resigned in May 2005 to care for his wife Geraldine, who had a serious illness.
* He is appointed Aston Villa manager in August 2006. Within months he is tipped as a potential successor to Sven-Goran Eriksson as manager of England but dismissed the speculation to stay with Villa. He guides them to three successive sixth place finishes in the Premier League from 2008 to 2010. Villa reached their first final in 10 years but were beaten in the League Cup 2-1 by Manchester United in February 2010.
* After four years at Villa, he resigns just five days before the start of the 2010-11 season.
* Joins Sunderland in December 2011, but, after a bright start, they fade towards the end of his first season to finish 13th in the table. O'Neill is sacked after 31 games of the 2012-2013 season with his side having won only seven matches.