O'Sullivan reveals selection headaches
O'Sullivan will unveil his 30-man panel for the trip to France on Sunday, the day after their World Cup warm-up Test against Scotland.
In relation to the back row, Leinster young gun Jamie Heaslip will start at number eight at Murrayfield, while Ulsterman Stephen Ferris gets a chance to shine at openside flanker.
Neil Best, who will wear number six on Saturday, is certain to make the cut, as are Simon Easterby, Denis Leamy and David Wallace, who have all been rested.
Mick O'Driscoll, meanwhile, should also get the nod because of his ability to play at both lock and flanker.
However, O'Sullivan is keen to evaluate the respective claims of Alan Quinlan and Keith Gleeson, both of whom have been named amongst the replacements for the trip to Edinburgh.
The back three would appear to be far more clear-cut but Brian Carney and Tommy Bowe, who will start on the flanks against the Scots, both know they need big performances this weekend to seal their places in the squad.
"This game is very significant for those borderline cases," O'Sullivan told reporters in Dublin on Tuesday.
"I always said selection would always come down to this game in certain departments.
"The back row and back three are areas that are up for grabs.
"I'm reasonably happy with the data I have on the locks; it's the back row that is causing me the greatest headache. We have a plethora of back rows to select from.
"Like in the last World Cup (2003), we have an embarrassment of riches in that department. I'm just giving myself one last chance to look at the back rows.
" I'm also having another look at Gavin Duffy (Connacht's utility back) at centre after he performed well against Argentina.
"These are outstanding issues in terms of the World Cup squad. Much as we want to win the game on Saturday, it's a means to an end. It will play a role in the final selection for the World Cup," he added.
And O'Sullivan admitted that Bowe and Carney are effectively playing for one place.
"Tommy and Brian are both good strike runners and footballers," he mused. "Tommy has been in the squad for a while now and he's a very capable player.
"We took a punt on Brian in taking him to Argentina but we what we saw was very positive and we have to give him another opportunity.
"There's nothing in between the two of them so Saturday is very important. It's hard to differentiate between them."