O'Leary first came to national prominence in rugby in 2004 when he was part of the Ireland team that finished as runners-up in the IRB Under-21 World Championship final.
Jamie Heaslip, who will also start at number 8 on Saturday, was one of O'Leary's Irish team-mates back then.
O'Leary's development as a scrum half has been steady since then but it really took off last season when he edged out Peter Stringer for the Munster number 9 jersey.
He started the Heineken Cup wins over Gloucester (quarter-final), Saracens (semi-final) and Toulouse (final) on the way to securing his second European medal.
The Dolphin clubman, a robust player with enough pace to also play on the wing, is happy with where he is at at the moment but would dearly love a run in the Ireland side.
"Since I've been professional, four or five years now, I've been working hard to improve my game. I think I've done that," O'Leary said.
"I suppose it's only in the last year that I've reaped most of those benefits (breaking into the Munster team).
"I'll be continuing to work hard and hopefully get a bit more reward in the future.
"For my first start for Ireland, this is the biggest test you could want. Playing the All Blacks at home, it's great.
"That's what you want as a player. You want challenges."
Speaking at the team announcement press conference, O'Leary admitted he had had little time to reflect on his selection for Saturday's Test and that he had not even spoken to his father or family yet.
Asked about what it means to him to get his first start, he said: "It's always nice when a coach has faith in you. As Declan (Kidney) said, he's building a panel of 30 or 40 players here so he has faith in all those players.
"I'm just lucky to get the nod this weekend. I'm not going to read too much into any selection.
"I'm just going to concentrate on the game on Saturday and concentrate on my performance - that's the main thing I'll think about over the next few days."
Between August 2007 and last summer's tour to New Zealand and Australia, three different scrum halves started Tests for Ireland - Isaac Boss, Peter Stringer and Eoin Reddan, with the latter being largely first choice.
O'Leary, now the fourth Ireland scrum half of recent years, knows he will have a battle on his hands to try and hold onto the jersey.
"With Reddser and Strings and Isaac Boss as well, there's great competition at 9 in Ireland at the moment.
"You can see that in the team selection there, every position is up for grabs at the moment. It's a great way to have it and it's up to us to made it hard for Deccie (to pick the team)."
On his return to Croke Park, O'Leary said it will be 'great to run out there in an Irish jersey.'
Great too to be given the chance to impress right from the start, with his only previous Ireland cap coming in the closing stages of a summer tour match in Argentina.
"I just came on five minutes from the end (against Argentina), on the wing. I think it was for an injury to Andrew Trimble or someone in the back-line.
"I was the last man in the bench, so I got five minutes on the wing at the end.
"I've been working hard to get this chance to start for Ireland and I'm just really looking forward to it," he added.
Moving from Munster to Ireland, Kidney has watched O'Leary's development with a keen eye and knows what makes him tick.
"Tomas has worked very hard. He's worked with several coaches through his scrum half career and he's been picking up bits and pieces from Dave O'Mahony, Alan Gaffney and Greig Oliver," the Ireland coach explained.
"Tony (McGahan) has been very good with him as well this season. He's learnt things along the way and he's had the ups and downs that every professional will have in his career.
"He has good attributes. I don't think, four days before a match, I should be trying to load him up with those (attributes). I think everybody has seen those and how well he can play. He's played well this season, as he did last season."
Kidney added: "Tomas is keeping out some exceptionally good players. Obviously I think highly of him.
"It was a rough evening last night talking to guys that are very good players and telling them they wouldn't be getting a start in a game which everyone was looking forward to playing in."