Head coach Warren Gatland questioned his players' pride after they lost the physical battle in last Saturday's 30-15 defeat by the All Blacks.
O'Brien says there can be no excuses in Wellington as the Lions look to salvage the three-Test series.
"It's attitude - we sat off and gave them too much respect," O'Brien said.
"They won [the physical] side of the game, there's no point in saying anything different, so that was obviously a big area for us where we let ourselves down.
"We drew a line in the sand during the week and spoke about the way we'd do things this week. It's about actions now."
Irishman O'Brien has been joined in the forwards by Maro Itoje and captain Sam Warburton as Gatland beefs up his pack before Saturday's match at the Westpac Stadium.
New Zealand have made two changes for the second Test - both in the backs - with Waisake Naholo coming in on the wing and Anton Lienert-Brown at outside centre.
They replace injured pair Ben Smith (concussion) and Ryan Crotty (hamstring).
Media playback is not supported on this device
Lions tour captain Warburton says there has been some wounded pride in their camp this week.
"You play rugby because you enjoy the physical side of the sport - that is definitely the case for me," the Welshman said.
"When you come off second best there, it does hurt you as a player.
"Sometimes you lose games from a bounce of a ball or a controversial decision but when you have been beaten pretty well from a physical side of things - and we were - that is disappointing from a playing perspective."
Warburton has called for a much-improved defensive effort from the Lions if they are to level the series.
"Our first three-phase mentality has to be better, it has to be aggressive," Warburton added.
"You can pick eight sevens [openside flankers] if you want, but if you don't win collisions, it's hard to slow ball down.
"It is a collision sport. Ninety-nine times out of 100, if you win most of the collisions, you'll win the game."
The British and Irish Lions must improve their attitude and mindset before the second Test against New Zealand, says flanker Sean O'Brien.