O'Leary and his then partner Stephen Milne did not qualify for the Star class medal race in Beijing, in which O'Leary won €3,600 after placing a €300 bet on 12-1 Britain.
The story came to light days before O'Leary and current teammate David Burrows began their Star class campaign at this summer's London Olympics, prompting an investigation by the International Olympic Committee (IOC).
But IOC chiefs announced yesterday that they had found "no proof of any match-fixing".
While Olympic athletes are banned from betting on Olympic events, the IOC's ethics commission agreed that O'Leary was not fully aware of the rules at the time.
"The athlete was unaware he could not bet on Olympic events," said IOC spokesperson Mark Adams. "It is not something we agree with and we condemn it but we will not take any more action."
O'Leary and Burrows finished 10th in the Star class at the London Games in what was a milestone summer for Irish sailing on world sport's biggest stage.