O'Leary placed two bets worth a total of €300 on the British boat to capture the gold medal in the Star Class at odds of 12/1, and won €3,600, as exclusively revealed in the Irish Independent last August.
"There was no proof of any match-fixing," said IOC spokesman Mark Adams. "The athlete was unaware he could not bet on Olympic events. It is not something we agree with and we condemn it, but we will not take any more action."
The IOC bars athletes from betting on Olympic events, but the organisation's ethics commission said competitors in 2008 may not have been as familiar with the regulations as they are now.
O'Leary failed to qualify for the medal race on which he had bet. The affair, which broke just before he competed at the London Games, made headlines and prompted an IOC investigation.