The majority of his 20-year playing career was spent as a central defender at Arsenal, where his tally of 722 appearances stands as a club record.

A calm and collected central defender, O'Leary was noted for his good positioning and elegant style of play.

[3] He missed the loss to Luton Town in the 1988 Football League Cup final due to an Achilles tendon rupture.

By this time, O'Leary was no longer automatic first choice (with the partnership of Tony Adams and Steve Bould at the centre of George Graham's defence), but he still turned in over 20 appearances as Arsenal won the 1988–89 First Division title due to a 2–0 win at Anfield on the final day of the season.

[12] O'Leary was captain in his final international game on 17 February 1993, but was injured in the opening minutes of the 2–1 friendly win over Wales at Tolka Park.

[3] When the former Arsenal manager George Graham was put in charge at Leeds United in September 1996, O'Leary was installed as his assistant.

Their 1999-2000 UEFA Cup campaign ended in the semifinal with defeat to the Turkish side Galatasaray, following the murders of two Leeds fans during violence the night before.

Although there was little indication of this at the time, this was a serious failure for the club because Ridsdale had borrowed £60 million against future gate receipts, budgeting for prolonged Champions League involvement.

[17] By June 2002, O'Leary had spent almost £100 million on new players in less than four years for no reward in terms of trophies, but he had never finished outside the top five as a manager.

[18] O'Leary's fame at Leeds rests upon his promotion of a series of younger players such as Woodgate, Bowyer, Alan Smith, Harry Kewell, Stephen McPhail, Eirik Bakke, Ian Harte, and Danny Mills (signed for £4 million from Charlton Athletic).

"[20] O'Leary was fired with two years remaining of his contract and sued for compensation as the club claimed he had left by mutual consent.

[24] In January 2004, O'Leary criticised chairman Doug Ellis for a lack of funding for transfers, as Villa had let go of 13 players while only signing Sunderland duo Thomas Sørensen and Gavin McCann for a combined £4.5 million; forward Dion Dublin had been playing in central defence.

O'Leary signed AC Milan's international defender Martin Laursen, Carlton Cole from Chelsea, and French midfielder Mathieu Berson ahead of the 2004–05 season, in which Villa finished 10th in the league.

[26] Despite six summer acquisitions including strikers Milan Baroš and Kevin Phillips, Villa finished 16th in 2005–06, just two places above the relegation zone.

Following the relegation of local rivals Birmingham City and Bromwich Albion, Villa were the only Midlands side playing Premier League football in 2006–07.

[27] On 19 July 2006, O'Leary's contract as Aston Villa manager was terminated by mutual consent, after an investigation into a statement against Ellis allegedly given by the squad.

[28] O'Leary returned to management on 4 July 2010 with United Arab Emirates club Al-Ahli Dubai, where World Cup-winning Italy captain Fabio Cannavaro was signed and made skipper.

[30] On 22 April 2011, Al-Ahli officially announced its decision to sack O'Leary with his assistant coach Roy Aitken.

[37] O'Leary's brother, Pierce, played for Shamrock Rovers and Celtic, and was capped seven times for the Republic of Ireland.

The scheme involved the loan of £266,000 (equivalent to £763,644 in 2023) by Arsenal, O'Leary's club, to trustees in Jersey, where it was held for his benefit.