After extending their unbeaten run to eight games by drawing at Southampton, defeat against Manchester United at Villa Park consigned them to sixth place.
The Villains finished five points short of a Champions League place and were edged out of a UEFA Cup spot on goal difference by Newcastle United.
Nevertheless, it was still a great achievement by O'Leary, who appeared to get the best out of the likes of Gareth Barry, Lee Hendrie, Jlloyd Samuel and 16-goal Juan Pablo Ángel.
The Colombian was the club's leading scorer, while Darius Vassell bagged 9 goals to cement his place in England's Euro 2004 squad.
O'Leary's side reached the last four of the Football League Cup by knocking out Wycombe Wanderers, Leicester City, Crystal Palace and Chelsea.
With the likes of Liam Ridgewell, Steven Davis, Peter Whittingham and the Moore brothers (Stefan and Luke) forcing open the first team door, David O'Leary had an abundance of young talent at his disposal.