The Tigers full-back won his 49th cap on Saturday in the defeat by Scotland, and will head for his first World Cup after a broken leg suffered in the final warm-up game in 2003 robbed of him of an appearance in that tournament.
O'Sullivan said: "Some very hard decisions had to be made. The Scotland game solved some of the selection issues in the back row and also the back three."
The 30-man Ireland squad now heads to Capbreton in the south of France for a five-day training camp, before returning to prepare for their final warm-up game against Italy at Ravenhill in Belfast on Friday, August 24.
Tigers players Martin Castrogiovanni (Italy), Marcos Ayerza (Argentina), Alesana Tuilagi (Samoa) and Seru Rabeni (Fiji) have already been confirmed in their respective national squads for the Rugby World Cup, while Aaron Mauger moves to Leicester once his duties with New Zealand have been fulfilled. The final national squads, including England's, are due to be announced tomorrow.
Ireland RWC squad:
Forwards: Neil Best (Belfast Harlequins/Ulster), Rory Best (Belfast Harlequins/Ulster), Simon Best (Belfast Harlequins/Ulster), Simon Easterby (Llanelli), Stephen Ferris (Dungannon/Ulster), Jerry Flannery (Shannon/Munster), John Hayes (Bruff/Munster), Marcus Horan (Shannon/Munster), Denis Leamy (Cork Constitution/Munster), Donncha O'Callaghan (Cork Constitution/Munster), Paul O'Connell (Young Munster/Munster), Malcolm O'Kelly (St Mary's College/Leinster), Alan Quinlan (Shannon/Munster), Frankie Sheahan (Cork Constitution/Munster), David Wallace (Garryowen/Munster), Bryan Young (Ballymena/Ulster)
Backs: Isaac Boss (Ballymena/Ulster), Brian Carney (Clonakilty/Munster), Gordon D'Arcy (Lansdowne/Leinster), Girvan Dempsey (Terenure College/Leinster), Gavin Duffy (Galwegians/Connacht), Denis Hickie (St Mary's College/Leinster), Shane Horgan (Lansdowne/Leinster), Geordan Murphy (Leicester Tigers), Brian O'Driscoll (UCD/Leinster, captain), Ronan O'Gara (Cork Constitution/Munster), Eoin Reddan (London Wasps), Peter Stringer (Shannon/Munster), Andrew Trimble (Ballymena/Ulster), Paddy Wallace (Ballymena/Ulster)