The Six Nations champions woke up to a barrage of criticism from the French media this morning after Argentina registered a sensational victory.
Their hopes of reaching the quarter-finals now hinge on their match against Ireland with defeat sending them crashing out just two weeks into the World Cup.
A far kinder opener awaits Ireland against Namibia in Bordeaux on Sunday night, but O'Sullivan's belief that Pool D will remain a ferocious battleground has only grown after last night.
"The occasion got to France a wee bit in the opening 20 minutes and they forced the game and made a lot of mistakes," he said
"I suppose it's hard to believe that France will ever play that badly again in the World Cup. It was just one of those nights that didn't go for them.
"To be fair Argentina had a lot to do with that, but I think the sense of the occasion and the pressure got to France.
"Having the home crowd behind you can be a good thing but it can also bring a little bit of extra pressure to bear.
"It was a poor performance that they will be disappointed with and they will expect to fix it very quickly.
"We didn't discover anything from the game that we didn't already know - we always knew the pool was going to be tough and that looks even more the case now."