O'Brien did not feature for Leinster as they defeat Glasgow Warriors on Saturday to clinch the Pro14 title, although he did lift the trophy in his final match for the club.
The 32-year-old has agreed a deal to join newly promoted London Irish in the Premiership following the World Cup.
But O'Brien will not be involved in the tournament in Japan, Leinster revealed on Sunday, as he requires an operation to fix his hip.