It was meant to be a walk in the park for the third-ranked Irish but it turned into a hard day at the office with the five tries to two victory not indicative of the outstanding effort put in by The Oaks.
"We had to pretty much engage in a slugging match which didn't suit us at all," O'Sullivan said of the blustery conditions and confrontational Romanian pack.
"It's hard to say we were rusty when we scored five tries - I think we defended really well."
Inspirational Irish hooker Keith Wood believes the Wallabies could be in for some softening up when they take on Romania at Suncorp Stadium on Saturday.
The Irish experienced the confrontational Oaks pack first-hand yesterday and Wood thinks Australia can look forward to an equally physical clash.
"I think they'll have a big impact on the group and they are fully able to take a game to the opposition and able to put teams under a lot of pressure. Just by the manner and nature by which they play there is I suppose an additional risk of picking up an injury."
Wood, 31, spent most of the past year out of rugby and played in just six of Ireland's past 24 Tests before today because of a chronic neck injury. But the man regarded as the outstanding No.2 in the game before his injury said it was time to move on.
"I think at this stage it's kind of got a bit long-winded talking about all that I've gone through," said Wood, who'll have his swansong at this World Cup.
"Injury is part of the game and I had no doubts I'd be back in time for the World Cup."
Wood did his bit yesterday by scoring a try and having a solid all-round game. He was replaced in the second half but didn't voice any concerns about a minor cork he received.
With a roaring wind at their backs in the first half, the Irish opened up a handy 26-0 lead courtesy of tries to Shane Horgan and inspirational captain Wood.
But Ireland flattered to deceive, with the Romanians catching them off guard with short kicks and aggressive play at the breakdown. A 44th minute penalty try to Romania's all-time leading try-scorer Gabriel Brezoianu put The Oaks back in the game and it was only some selfish play by winger Cristian Sauan that stopped the outsiders putting on another five-pointer just five minutes later.
In the end the class of the Irish showed with a double to winger Denis Hickie and a try to tireless flanker Victor Costello proving more than enough.
Outstanding Irish centre Brian O'Driscoll, who took a heavy knock in the first half, had his moments but failed to break a try-scoring drought that has lasted almost 18 months.
Wood look-alike, prop Petru Balan was outstanding for the Romanians before being replaced midway through the second half. He was later taken to hospital with a leg injury and may be in doubt for the game against Australia on Saturday.
Romania coach Bernard Charreyre would not single out individual players, praising his entire squad but lamenting the final margin which he thought should have been about 10 points.
"We are disappointed about the score but we have a lot of hopes about the future of Romanian rugby," he said."I am proud for all the players that played tonight. They were able to create a lot of problems for the Irish team - one of the best in the world."
Fullback Girvan Dempsey showed much in attack for the Irish while Horgan offered good support on the wing after a long injury lay-off.
The five-try win earned Ireland a bonus point and lifted them to the top of Pool A where it also tussles with Argentina, Australia and Namibia for two quarter-final berths.