Warburton, who has played little rugby recently due to injury, put in an improved performance under the roof in Dunedin compared with his opener against the Provincial Barbarians, but it seems Gatland is fully prepared to make the pragmatic decision to leave him out should O'Brien's form convince him to.
"He's well aware of the competition that's there at the moment," Gatland said of the Welshman. "He fully understands that loose forward trio went outstandingly well against the Crusaders.
"One of the reasons why we selected him as captain of the squad is that he's a quality player but... this tour isn't about Sam Warburton, it's about putting the squad first. If he's not involved in the first test because of the performances on Saturday night he will fully understand that.
"The challenge for them [starting loose trio] is do it again, and if they do it again then we will sit down and make what we think is the right selection for the test matches."
Of the importance in striking the right balance among his back row, Gatland said: "One area that has been going well for us is our lineout... we've had a few hiccups but we know that one aspect of Peter's game that he's exceptional whether that's in attack or defence. It's trying to get that balance right... Toby is obviously very good in the air too and at lineout time."
The rest of Gatland's starting XV could almost name themselves, with Vunipola starting after starring in Christchurch, and halfback Conor Murray also being repaid for his work at AMI Stadium.
One of the few question marks surrounds where Gatland intends to play Owen Farrell in the first test.
Farrell has been named on the bench for Saturday, with Johnny Sexton at No10 and Leinster teammate Ben Te'o outside him at second-five, but Gatland wouldn't rule out the possibility of moving Farrell to second-five during the match. If so, that will be a likely indicator of where the Englishman will play against the All Blacks.
Of Steve Hansen's team to play Samoa at Eden Park tomorrow, Gatland said: "It's close to their test team, I think. It looks like their strongest side that they've put out. They're trying to get a game under their belts. I'm not sure they'll make many changes from that game against Samoa to the following weeks."