But O'Brien, known as the Tullow Tank for his ferocious ball-carrying ability, is determined to force his way into contention to face the Wallabies.
"Sam is captain but, in any tour, if someone isn't performing or if he picks up a knock, there are opportunities there," said O'Brien.
"Gatland knows him from being involved with Wales and he's led them pretty well, led them to a lot of success so he's a world-class player.
"But I'll be pretty determined to make the Test side. I'm going to back myself against whoever is out there."
The back-row is arguably the most competitive position in Gatland's 37-man squad and there were some high-profile casualties at Tuesday's announcement.
But of those that made the cut, O'Brien is battling Wales quartet Justin Tipuric, Dan Lydiate, Toby Faletau and Warburton, Ireland teammate Jamie Heaslip and England's Tom Croft for the starting spots.
One feather in the Leinster's star's cap though, is the ability to play effectively across the entire back-row.