The other five players - Geordan Murphy, Leo Cullen, Shane Jennings, Eoin Reddan and Peter Bracken - were all involved in the Heineken Cup final with their respective clubs, Leicester Tigers and London Wasps, and are due to link up with the squad in Argentina tomorrow.
Speaking to the media after training, O'Sullivan is looking forward to seeing what some of Ireland's up and coming players have to offer.
"Everybody has to play well now in these last two games because there will be no future opportunity, as there is only one more camp before we're naming the World Cup squad.
"For sure, we have fifty percent (of the squad in mind), which is the players we left home. But there are 30 players more here.
"It will be an opportunity for both ourselves and Argentina, for the players to be in the World Cup squad, so I think both teams will play well on Saturday.
"We'll have to position well for set-pieces and keep our discipline."
The clash at Santa Fe's Brigadier Estanislao Lopez del Barrio Centenario stadium will see Ireland get an early look at the Pumas, their World Cup Pool D opponents.
"It probably would be better if we didn't have to play each other (over the next two weeks)," admitted O'Sullivan.
"But I think the best way we have to deal with this is selecting some of the younger players to see if they are ready to fight for places in the World Cup squad."