Prop John Hayes is set to make his comeback against Romania in Saturday's World Cup opener in Australia. Hayes has not played since March because of a calf problem but O'Sullivan is keen to get him back on the field as quickly as possible.
"He has to have a first game back sometime and he is on track to start next week," O'Sullivan told a news conference on Monday. "Match fitness is a bit of an issue and frontrowers tend to need game time, but he is still our starting tighthead and he is going to be crucial".
O'Sullivan said he would have a full compliment of players to select his team from even though prop Marcus Horan failed to complete Monday's training session. "Marcus Horan has a slight strain in his quad but is no concern, he should be fine. We just took him off as a precaution," O'Sullivan said.
The focus of Pool A is naturally on the games between Ireland, Argentina and Australia, with only two able to qualify for the quarter-finals. While O'Sullivan is looking to build through the pool stages until their final two crunch fixtures, he nevertheless vowed to play his best possible side against the Romanians.
"If you get your gameplan right you have a chance of beating them. They are a team that we know what it takes to get past them," he said. "We played them last year and it was a very difficult game because they are quite a physical team.
"They all play their professional rugby in France, so there is a French influence on the team. They have a very strong pack of forwards, strong-running backs and they are very good at offloading in the tackle "They are a side that is well put together."
The Irish Squad will be available on this site as soon as it is announced.