Pre-tournament favourites Ireland and France have both failed to produce consistent performances in their four games to date, with both sides winning three and losing one.
Ireland opened their account with a hard-fought win over Wales in Cardiff before going down to a last-minute try at home to France.
They then hit peak form with an emphatic 43-13 victory over England only to struggle a week later against unfancied Scotland.
Meanwhile, France impressed in their opening win over Italy, which they followed up with a key victory in Dublin.
But they were then less-than convincing in the win over Wales in Paris before going down to England at Twickenham last weekend.
O'Sullivan believes that the peculiar demands of the Six Nations has made it difficult for his side to reproduce the form that saw them convincingly defeat South Africa, Australia and a makeshift Pacific Islands team in the autumn.
"It would be hard to argue that except for the England match we have hit the levels we set in the autumn," admitted O'Suullivan.
"We have to factor into that there were only really two games in two weeks during the autumn. The Six Nations is five matches in seven weeks.
"It's a different tournament and the intensity is higher than the autumn internationals, even if you are playing sides from the southern hemisphere.
"The intensity of Six Nations rugby is underestimated in most parts of the world. It's fairly bruising rugby for every side.
"Defences are tighter and more aggressive. It's hard to play well in the Six Nations."
O'Sullivan also cites the example of France to help explain how difficult it is to maintain top form throughout the Six Nations.
"Look at France - they were going to win the World Cup a couple of weeks ago. They were on track for the Grand Slam and everything was hunky-dory," O'Sullivan added.
"Then they lost at Twickenham. Where did that come from? The bookies didn't see it either and that's Six Nations rugby.
"I've said it all the way through. Of course the ambition is to play five perfect games and win the Grand Slam but it rarely works like that.
"You have to be realistic and know that some days you'll have setbacks when things don't go right. But you have to move on as it's the process of building.
"For that reason I'd say our four games in the Six Nations have not been as smooth as the two in the autumn in terms of performance, but it's a different landscape.
"I'm not in a panic over it. We're not too far from where we want to be. If we win on Saturday with a good performance, we'll have had a good solid Six Nations."
O'Sullivan's side face Italy in Rome on Saturday with a good chance of winning the Six Nations title.