The Munster scrum-half fractured his ankle in his side's Magners League win over the Scarlets as what had been a thrilling season came to a disappointing end.
Nearly five months on the sidelines followed for the 25-year-old, who was forced to watch the Lions' adventure from a distance knowing he should have been involved in the thick of the action in the Republic.
There was good news all round this weekend, however, as O'Leary finally stepped back on to a rugby pitch three weeks into the new season and a month-and-a-half out from Ireland's Autumn internationals against Australia, Fiji and South Africa.
"I'm absolutely delighted to be back and it's a great honour to be playing for Munster again," said O'Leary after he featured in the last 15 minutes of Munster's dramatic late win over, ironically, the Scarlets at the weekend.
"I suppose through any adversity you do come back stronger. Hopefully it has made me a stronger player and person.
"I realise you will get your fair share of injuries through a rugby career. Some players are lucky to go throughout their career without a major injury and some people aren't. It's part and parcel of the game.
And although he admitted the state of his ankle had caused some sleepless nights and had preoccupied his mind in the lead up to Saturday's victory in Llanelli, O'Leary felt no adverse affects either during or after the 22-20 win at the Parc-y-Scarlets.
"The ankle felt good and I wasn't thinking about it when I was playing - that's a good sign," added Ireland's Grand Slam winning No9.
"I've waited a long time and worked really hard to comeback but I was glad to get through the last ten or 15 minutes of a game.
"I have been wary about the injury in training over the last few weeks but, once you're on the field, the game takes your mind off it. I put in a few hits and there were no problems.
"I never thought I wouldn't make a return. The first few weeks were tough but the surgeons and physios in the USA and at Munster have been great.
"Hopefully playing will give me some confidence moving forward."