This year they open the championship with a daunting trip to the Stade de France on February 14 while they meet Sir Clive Woodward's Grand Slam heroes at Twickenham on March 6.
"The Six Nations is a a benchmark for an international season but what you have to remember is that it changes every year, for every team," said O'Sullivan.
"We had a good draw last year and got off to a good start in Scotland and Italy. We then went on to beat France in Dublin and that gave us plenty of momentum which set us up for Wales and England.
"The whole momentum of the tournament suited us, but this year is completely different. Our first game is in France, the second is at home to a resurgent Wales side while the third is against England at Twickenham.
"To generate the same momentum this year as we did last year is going to be a lot more difficult."