Man-of-the-match O'Brien helped the Irish set up a mouthwatering showdown with Wales in Wellington on Saturday and then reminded his team-mates they have won nothing yet.
"We feel good. We've done our job and topped the group, but this squad of players isn't happy with that," said the Leinster man, who was at his rampaging best at Otago Stadium.
"We want to do bigger and better things than this. There's way more to come from this team.
"We said after the second pool game we're halfway there and now we'll say again, we're only halfway there.
"There's no point thinking we've achieved something now because we've done what we've done.
"Obviously it's good, but we still want more and Wales is next on the list. Hopefully we can take it up a notch.
"There's a big week ahead of us and we'll certainly be holding nothing back."
Defeat to Italy would have seen Ireland return home tomorrow, but they rose to the occasion magnificently by thrashing the Azzurri out of sight.
"It's tough to come into a game knowing that nothing other than a win will do," said O'Brien.
"We stuck to our guns and our gameplan and it paid off. We never strayed from what we meant to do.
"There's a great mixture of youth and experience in the squad. The experienced lads you follow in big games like this.
"It was a good game. It was physical and fast - everything you wanted.
"We were all about being direct in this game and things opened up for us."
O'Brien spent several seconds bare-chested as he changed shirts, resulting in a chorus of whistles from the crowed, but denied it was his Sonny Bill Williams moment.
Chiselled New Zealand centre Williams grabbed the headlines when he took off his jersey in the World Cup opener against Tonga.
"I don't think I'm in as good a shape as Sonny Bill Williams!" he said.
"I was trying to get the shirt on as quickly as possible. I was struggling to get it on.
"I'm weighing in a bit heavy at the moment."
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