Now, at the age of 32, he is back in the GUINNESS PRO12 and battling it out with Ireland's first-choice No.9, Conor Murray, for game time at Munster.
And O'Leary hopes to impress in that pursuit enough to catch the eye of Ireland head coach Joe Schmidt.
"I'd love it, but I guess my main job here is to put as much pressure on Conor as possible," he told the Irish Examiner.
"If I can even make the coaches start thinking about who they should be selecting at nine, then I might get into the shake-up for the green jersey again.
"If I can express myself down here, then you'd never know, they might end up picking two Munster scrum-halves at some stage.
"You look at Redser (Eoin Reddan) and Bossy (Isaac Boss), they're both 35 or something. I'm 32 so there's scope for a player of my vintage for a few more years.
"Touch wood I can go for another few years because I am really enjoying it at the moment, I think I have a lot more to offer Munster and Irish rugby."
But O'Leary, who lifted the European Cup twice during his previous spell with Munster, revealed his return to the province was not always guaranteed.
"I had a few fantastic years in London," he added. "It was a different experience, different lifestyle, something new that gave me a chance to step back from the rugby.
"When I left initially, I wasn't thinking about coming back, but I guess when I was watching the big European games and the lads playing against Leinster and stuff, it made me sit up and think: 'Jeez, I'd love to be there'.
"It made me appreciate how special Munster is and in the last year of my contract, I was definitely considering it.
"I'm back and quite happy with my own form. Whether I'm on the bench to Conor or starting, I want to play and try and express myself as best I can. So far, so good and I'm really happy with how I'm going."
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