Ireland play Scotland in a warm-up game at Murrayfield on Saturday week, with O'Sullivan announcing his 30-man panel for the tournament the following day.
Asked today about his reasons for waiting until after that game to name his final squad, the Corkman said: "The Scottish game is playing a part in selection - that is why we are waiting until after the game to name the squad. It is good to have everyone involved for a while.
"I don't think it would have been a good dynamic to name the final 30 a month or two ago. The guys who were not in that 30...well there wouldn't have been much motivation for them to get stuck in.
"To be fair to the lads, all of them have worked really hard in the last few weeks, both in Poland and back at their bases and this week they have worked extremely hard."
O'Sullivan gathered his extended squad together last Sunday in Limerick for a week-long training camp and has been pleased with what he has seen. He intends to reduce his squad considerably tomorrow from 48 players to "between 36 and 38".
"They are all intelligent guys. Some of them know that the odds of a month in Bordeaux are slim but they still pitched in and worked their guts out. I think if they knew in June then there would be less motivation for them.
"It makes sense to wait until the eleventh hour, we are not going to lose anything by waiting and we will certainly be wiser when we do make the decisions."
While some players may be already assured of their places in the World Cup squad, O'Sullivan was quick to point out that there are "some places still up for grabs." "There are some places still up for grabs, that will affect selection for Scotland - in the back five and probably in the back three as well. Also, as we look at the build-up, we have the two formal games and a probable game down in Capbreton - that is not confirmed yet, though I hear that a referee has been appointed.
"There is a chance that the guys playing against Scotland wonâ00t play against Italy and vice versa and probably a mix of the two in Capbreton. Each game has its purpose and will allow us to hit the ground running."
So will he be experimenting with the selections for the upcoming games?
"You need to park that notion of experimenting," O'Sullivan said.
"Experiments are over. What we are now looking at is nailing down a few places in the squad â00 thereâ00s not a whole lot of them at this stage as you would expect.
"Once we get into the tournament, every time we donâ00t pick our first fifteen you canâ00t say we are experimenting. We are in a tournament situation so we are going to try to pick teams that we can put on the pitch that can play together.
"We are trying to build a squad and combinations that are interchangeable and the selections will reflect that. We know what we have. The only issue is that there are a couple of spots in the back three and back five."