The Italians almost shocked Ireland in a World Cup warm-up game at Ravenhill last August and O'Sullivan knows only too well the threat they can pose.
Speaking on RTE Radio 1, he said: "It's a very difficult game to start with. We've had more than our fair share of starts with Italy. They always come out the first day of the tournament with all guns blazing and they throw everything but the kitchen sink at you because they're a hugely physical team.
"They've a new coach involved now, Nick Mallett has taken over so we're not quite sure what to expect from them in terms of technical and tactical stuff.
"But we know one thing, it's going to be a bruiser of a game and it always is (against Italy). They are probably the most physical pack in the Six Nations."
Last March Ireland ran in eight tries against Italy for a memorable 51-24 victory at Stadio Flaminio which saw them just fall short in their attempts to deny France the Six Nations title.
An altogether closer margin is expected in Dublin next month though, especially with the Azzurri being known for their good starts to the tournament - they beat Scotland in their 2000 opener and also accounted for Wales in their first game in 2003.
"They've caught teams out on the first day of the tournament. They've beaten Wales and Scotland a couple of times. They tend to put their best foot forward on the first day of the tournament," O'Sullivan added.
"We're under no illusions about this game. It will be a bruiser of a game and a tough one to start with."
The 2007 Rugby World Cup has plenty of bad memories for O'Sullivan and his squad but there is a positive outlook in the Irish set-up that the Triple Crown winners can turn things around, beginning with next month's return to Croke Park.
"Our record in the Six Nations, over the past few years, is very good. We've lost only two more games than France over the last four years and we're well ahead of the rest of the pack," the Corkman explained.
"It's a competition we focus on ever year and like to do well in, and by and large we have done well in the Six Nations over the last few years.
"It's about focusing on that now. We can't change what happened in the World Cup. We were all dreadfully disappointed - the supporters, the players and the management. We didn't deliver anything like the potential we have.
"The key now is to refocus and get back to where we were during the last Six Nations and deliver another big Six Nations, which is all we can do.
"Talking to the players before Christmas and since then, that's their mindset at the moment. I'm very positive about the Six Nations and looking forward to it."