The Leinster number 15 played against Scotland at Murrayfield last weekend and completed the full 80 minutes despite a groin strain.
The Irish got off to a flying start in the RBS 6 Nations campaign with a 36-6 victory in Edinburgh.
O'Sullivan said: "Girvan finished the game fine at Murrayfield. There were no problems.
"But on Sunday night it became sore again and it was the same on Monday. We expected him to be okay but we're taking John Kelly as a precaution.
"He'll fill in for Geordan Murphy if Girvan doesn't make it with Geordan going to full-back. But we won't make a decision until Friday."
O'Gara's injury setback took O'Sullivan by surprise but gives Murrayfield hero David Humphreys a chance to build on his outstanding performance against Scotland when he scored 26 points.
Harlequins' Paul Burke retains his place on the bench.
O'Gara missed last weekend's match with an ankle injury but his recovery has given rise to O'Sullivan considering recalling the Munsterman who has been his first-choice number 10 this season.
"We expected Ronan to be fine but at training yesterday he was running and there was a reaction," said O'Sullivan.
"It was a bit of a surprise as we were quite confident he'd be fine.
"But he saw the specialist this morning who said that while there was nothing structurally wrong with the ankle it needed more rest."
Leicester star Murphy is the only change from the team that beat Scotland, replacing Shane Horgan on the right wing, having come on for the Leinster man early on against the Scots and scoring a second half breakaway try.
Centre Rob Henderson is promoted to the bench after completing his comeback from injury in the A side who drew with Scotland last week in Stirling.