The 32 year old midfielder tied up the deal earlier today and will link up with former Swans assistant manager Kevin Nugent at Brisbane Road.
O’Leary has made over 300 appearances for the Swans since making his senior debut in 1995, scoring 10 goals in the process.
He did spend a month on loan at Cheltenham in 2006 with the Robins keen to sign the Welshman on a permanent contract, but he rejected Town’s offer, instead opting to sign an extension at the Liberty Stadium.
“It’s been frustrating, but Orient have five fixtures throughout September, so hopefully I will get some game time,” O’Leary told the Swansea official site.
“I obviously know Kevin (Nugent) from his time at Swansea as a player and assistant manager, so it will be nice to link up again.
“At the moment I’m not looking beyond the initial month. We’ll just see where we are at the end of September.”
