The Irish provinces cruised into the quarter-finals with two thumping victories over English clubs, Munster crushing Premiership leaders Sale 31-9 and Leinster dispatching pool winners Bath 35-23.
The emphatic nature of the results gave Irish rugby a welcome shot in the arm just two weeks before the RBS 6 Nations starts, but O'Sullivan is wary of comparing the European Cup with Test rugby.
"It's great that Munster and Leinster are flying the flag for Ireland in Europe. The fact the players are confident and there is a lot of competition in the squad bodes very well," he said.
"But the Six Nations is a different tournament to the European Cup. When we're in Paris or Twickenham there will be 15 internationals against us.
"It's a different level. The European Cup goes up a notch every year, but it's not Test rugby. In my view Six Nations rugby is as high a standard as you can get."
O'Sullivan's caution in adding to the hype surrounding Munster and Leinster is understandable, given Ireland's anti-climatic showing in last year's RBS 6 Nations.
Ireland were favourites to win the tournament and were even touted as Grand Slam contenders only to crumble under the burden of expectation, finishing third with defeats against France and Wales.
The team which had delivered so much the previous autumn with splendid victories over South Africa and Argentina appeared in need of a major overhaul just four months later.
They were not helped by a series of injuries to Brian O'Driscoll, Gordon D'Arcy and Shane Horgan - with the three centres' absences causing havoc in midfield.
Suspect temperament has been offered as an explanation for last year's RBS 6 Nations failure - but O'Sullivan believes it was these injuries which derailed the Emerald Isle's chances.
"In 2004 we had a very strong showing in the autumn against South Africa and Argentina. People were upbeat about Ireland's chances in the Six Nations, and rightly so," he said.
"Things looked rosy, because it was a settled team. But for me the losing of the championship was based on injuries. We lost O'Driscoll for one game, Shane Horgan for two and Gordon D'Arcy for five.
"That dismembered our midfield and was our undoing. If we get a clean bill of health and keep our key players healthy we can give anyone a rattle, and that's the nature of it.
"We have a clean bill of health at the moment, which is excellent news."
O'Driscoll remains crucial to Ireland's chances, so his form for Leinster on his return from shoulder surgery - topped by last Sunday's masterclass against Bath - is hugely pleasing for O'Sullivan.
"Brian has come back from a very bad injury. It's taken a while but he has three games under his belt and has got better with every match," he said.
"He's worked very hard over the last six months to get it right. It's hard to imagine him getting better than he was on Sunday - but if he keeps that form up we'll be very happy.
"He's right up there where he wants to be."