But throughout his international career - across which he has now collected 27 caps - O'Brien has proved himself more than capable at blindside, or at No.8 where he will be battling Leinster teammate and Ireland captain Jamie Heaslip.
And O'Brien admits he's not bothered about which shirt he wears, provided he can make it his from the start.
"I'm going to back myself against whoever is out there and that's always been the way I've been taught," he told the Irish Independent.
"I'm not going to say, 'oh I can't play 8 because the two lads are there', so obviously there are options there all over the place. I played No.8 against Italy a couple of years back for a while, so Declan Kidney knew I could play 8 as well.
"Obviously, with Jamie being so consistent, he's very hard to move and I suppose our partnership as well, with me being in the same back-row as him, has worked pretty well. I suppose that was looked at by the previous Irish coach but I can leave myself open to play in any position.
"It wouldn't be my view to be an impact player, no, it would be my view to start and that's what I'll be hoping for. I can play all three and that's the way I'm going to stay."