For now, Nolan sounded most intrigued by O'Sullivan, who started Friday and delivered the biggest spark among the three candidates.
O'Sullivan completed 5 of 8 passes for 71 yards and looked in sync with rookie receiver Josh Morgan.
But his compelling case was tainted by an interception and a fumble, both coming on chances O'Sullivan didn't need to take. "J.T. O' Sullivan did a very good job, if not for those two turnovers,'' Nolan said. "But those two turnovers did occur. You have to take everything into account."
O'Sullivan had moved the 49ers 56 yards on seven plays before chucking a 35-yard pass over everyone but cornerback Stanford Routt.
He later fumbled while trying to scramble for the marker on third-and-six.
O'Sullivan also ad-libbed a backward pass to running back DeShaun Foster, a play that backfired for a 4-yard loss.
Still, Nolan sounded as if he found something to admire in the miscues.
Asked about the Foster play, he said the ill-advised gambit was the result of O'Sullivan's competitive nature. "It's like Brett Favre trying to make a play,'' he said.