He carried hard and often. He tackled hard, he cleaned out and he made things happen. It was a performance impossible to miss: a performance that enabled O'Brien to scream out that he's pushing hard to start in the first test.
His energy, dynamism and accuracy were infectious and he combined neatly with fellow loosies Peter O'Mahoney and Toby Faletau.
As a unit, the Lions' loose trio were tight, bruising and influential. "A lot of people put their hands up and had some pretty good performances, there is no doubt about that," says Gatland.
"The players on Tuesday night know they have to go out there and perform as a group and give a good performance so that they are in contention. I don't think there is any rocket science behind the performance of the team and some of the individuals in it. The other players in the squad are aware of that."
The pressure is most certainly on Warburton, to play with the same sort of impact as O'Brien.
It's not that Warburton can't reach that level - he's proven himself to be world class over the years. He just hasn't been able to do it recently or on this tour and form is surely going to count for everything when it comes to picking the Lions test team.
Warburton is short of football having been injured ahead of the tour and managed 65 minutes in the opening game against the New Zealand Provincial Barbarians before he tweaked an ankle that prevented him from being involved in Christchurch.
That injury has also curtailed his training load and the plan, it would seem, is that Warburton will start on Tuesday and probably again on Saturday against the Maori.
The coaching panel will be keen to give their captain every chance to make a statement and Warburton accepts that he will have to make it.
The competition for places is so fierce that he knows being captain won't save him from missing out.
"Over the years people have asked me who I think are the best sevens and I have always said that if I had to name five I would put Justin [Tupric] and Sean. It was the same going back four years ago when there was tremendous competition between us.
"I thought that going into this tour that there were eight boys in the back row and I thought any three of them could be picked for the test team. I think they are all that good and it is a nice luxury for the coaches to have and it is great competition for us as players."
