"I'm not fazed by wherever I start. As long as I'm playing, I'm happy," he admitted, ahead of his full RBS 6 Nations debut and only his second start in an Ireland senior jersey.
O'Brien, who turns 24 on St. Valentine's Day, began his rugby at Tullow RFC in Carlow where he still finds time to coach in between matches.
His potential was quickly recognised with a run at Ireland Youths level and Under-20 honours followed in 2007 when he was part of Eric Elwood's Grand Slam winning side.
"I took a lot of pride coming up through the (Youths) system and there are a lot of good players that are in the system at the minute who came through that as well.
"It meant a lot to me, staying with my home club and coming the whole way up through the ranks. I had self belief. I was very determined and I was stubborn, but I suppose it grows as you get to the Academy and you learn very quickly."
The Under-20 success of four years ago must seem a world away now that he has become a mainstay in the Leinster team and forced his way into Declan Kidney's starting line-up for the Six Nations.
O'Brien, whose one and only previous start was against Samoa in November, added: "The start of a Six Nations Championship is special and I'm really looking forward to getting out there and playing.
"I played once (during the autumn internationals) and obviously you are disappointed if you're not picked. You have to be really patient. I didn't want it to affect my training at the time, or going back to Leinster.
"I just kept my head down, kept doing what I was doing. I wouldn't say I felt left out. I was still involved, I was still training. It wasn't as if I wasn't in the squad.
"It's just a thing you have to get on with and one of those decisions. You have to go along with the coach and row in behind him."
With the current injuries to Jamie Heaslip and Stephen Ferris, the clash with Italy represents a good chance for O'Brien to perform and lay claim to a starting place for the weeks to come.
"It is an opportunity this weekend to get in there and hopefully keep a place. The lads (Jaime Heaslip and Stephen Ferris), I don't know when they will be back - but at the same time they will be back sometime and it's up to me this weekend."
While the Samoa match gave him another taste of Test rugby, O'Brien readily admits that he struggled to have an impact on what was a scrappy contest at the Aviva Stadium.
"I think that day it was never going to be a flash game. Looking back on the game myself, my own review, I didn't think I had a bad game," he explained.
"It was just a battle. It wasn't very open. I wasn't making line breaks. It was never going to be that kind of game.
"But I suppose if things were different then, I could have gotten another shout (in the following games) if it was a bit more open of a game. I have a point to prove coming into this one."
He then sat down with the coaches and went through what he could improve on. He will face one of his biggest tests to date on Saturday, with Italy captain Sergio Parisse being his direct opponent. O'Brien considers him a class act.
"Parisse is a world class number 8. He's everything really. He is a good lineout operator. He is fast, has good feet, he's strong and it will certainly be a good test (of) where I am coming up against him at the weekend.
"I'm really looking forward to it and I just can't wait to get out there. You really do know what you will get when you go over there (to Rome).
"Even the last games that they have played - they really went out and went after teams, so we are going to start fast and first things first, is going to be the physical aspect of it. That will be a massive influence on the game."