The sailor, who did not qualify for the medal race on which he gambled, won €3,600 after he backed the British boat to win gold in the Star class.
A statement on the IOC's website read: "The EB (Executive Board) decided to issue a warning to Irish sailor Peter O'Leary who admitted betting on an Olympic sailing event at the Beijing 2008 Games, but denied any competition fixing.
"No proof of any match-fixing of the competition in question was found.
"In addition, at the time of the Beijing Games, the IOC had just begun its campaign to educate athletes on the risk of irregular and illegal betting, and it seems that, in this particular case, the athlete was not aware that he could not bet on an Olympic event.
"While the EB views any breach of its betting rules as a serious matter, the EB considered the special circumstances of this case when taking this decision."
The IOC investigation into O'Leary's bets was launched this summer after news of the incident broke just before the sailor competed at London 2012.
O'Leary, along with team-mate David Burrows, finished 10th in the Star class at the Olympic Games in London.