Should Ireland win, O'Sullivan believes his side could go in to their remaining matches against Scotland and then England with the crown within their grasp.
"Next weekend would have been a pivotal game in the RBS Six Nations regardless of whether we had won or lost in Paris," he said. "It is the middle game and entering it with two wins as opposed to just one can define your Six Nations, no matter whether you are England, France or Scotland.
"If we get a result on Sunday we go into the Scotland game with everything wide open for us. It is similar to two years ago when we lost in Paris, our second game was against Wales and then our middle game was against England.
"We beat England and that turned our championship in the right direction."
O'Sullivan, says the absense of Wales Captain Gareth Thomas will be a major blow for the current Grand Slam champions.
"Gareth is a world class player and is a loss to them. We have lost Paul O'Connell to injury so I believe that's a fair trade off in the circumstances," he said. "He was superb on the Lions tour - he is a great captain and a very charismatic guy. I wish him well. The sooner he gets back the better for Welsh rugby. He has been hugely influential for them."