A highly-rated defender at Arsenal for 18 years and more than 500 games, he won two League Championships, two FA Cups and two League Cups domestically.
He was also a seasoned international, earning 67 caps for the Republic of Ireland.
At Italia 90, O'Leary turned unlikely goalscoring hero for his country when his penalty kick booked a surprise place in the quarter-finals.
He retired as a player three years later after moving to Leeds United.
But then began an acclaimed managerial career at the Yorkshire club, peaking in 2001 with a semi-final spot in the Champions League.
A respected analyst of the game, O'Leary joins the BBC television team for World Cup 2002 alongside former international team-mate Mark Lawrenson.