The 22-10 result leaves Ireland just a point adrift of Australia ahead of the sides' much-anticipated showdown at Eden Park next Saturday.
Given Ireland's recent form and the fact that current team manager Paul McNaughton was part of the last team to beat Australia outside of Ireland in 1979, O'Gara says that they will enter the game as 'massive underdogs'.
"We have a lot to prove and I think we have a lot to prove to ourselves. The big key for us this week is to get it right and we're aware we have great support," he said, with reference to the travelling support and ex-pats in the 20,823-strong crowd.
"The fans were unbelievable tonight, it was fantastic and we need to reward them. Next weekend is a key game. I think we're looking for the performance first of all and then the result will be further on in the game."
Ireland's record points scorer continued: "It's completely, completely different. We nearly lost that game. There's a completely different mood in the camp.
"Guys are quite happy. Obviously they're disappointed about not putting up a big score tonight (against the USA). But they're quite happy to put that behind us and move on.
"Obviously we've a huge game next week. The mood in the camp is completely different. I didn't feel under any pressure out there.
"We gifted them a try when we were chasing a bonus but before that they didn't look like scoring a try at any stage. On a dry day, I think you'd put 50 or 60 on those guys."
Ireland struggled to build a match-winning lead in New Plymouth, with World Cup debutant Sexton missing a brace of penalties in each half.
Sexton's off night with the boot added to speculation that the place-kickers at the tournament are encountering problems with the Gilbert Virtuo ball. Jonny Wilkinson had some well-documented difficulties against Argentina too.
This is nothing new at World Cup level. There were concerns about the Gilbert Synergie ball four years ago after New Zealand's Dan Carter missed five out of nine kicks against Scotland and there was also an issue about match officials over-inflating the balls.
O'Gara admits that missed kicks just happen in the pressure cooker atmosphere of a big tournament. "Mike Ross asked me (if there was an issue with the ball) in in the dressing room, and no, there isn't," he explained.
"I suppose it's pressure. I've watched Jonny Wilkinson for 13 years and haven't seen that ever before. I was talking to Jonny (Sexton) inside and we go all through it.
"Some days it's very easy and other days it's so difficult, that's the beauty of being a goal-kicker. You don't know what's involved and there are very few of us understand what it takes.
"He's just got to wipe the slate clean form his point of view and move on. I've been there before and that's exactly what you need to do, just don't read too much into it.
"World Cups come once every four years, they are the biggest (tournaments) and pressure gets to people in different ways. It can happen to any of us."
O'Gara will be hoping to get the nod over Sexton for next weekend's crunch encounter with Australia. The team announcement will be made on Wednesday (1.15pm local time/2.15am Irish time).
Asked about the the tweaks needed for Auckland, the Corkman added: "We were probably looking for a miracle pass here and there. I said to Paulie (O'Connell) inside there, you get flashes of genius in international rugby in one of three games.
"You just have to keep wearing teams down, keep holding onto the ball and going through it and then putting over a fella with just simple hands at the end of 15 or 16 phases, or else a penalty results.
"We're probably looking to come up with big plays too frequently and we've just got to get back to basics and hold onto the ball and keep our error rate down."
Follow the Ireland team in New Zealand on www.twitter.com/irfurugby.