Nolan said Monday that O'Sullivan will start Thursday's exhibition game against the Bears in Chicago. The sixth-year journeyman, with his eighth NFL team, has emerged as the team's top quarterback in a summer competition with veteran holdovers Alex Smith and Shaun Hill.
For most NFL teams, the third preseason game usually is a dress rehearsal for the regular-season opener. But Nolan said the 49ers still have not made a final decision on their starting quarterback for their Sept. 7 opener against Arizona.
"I feel good about the (quarterback) rotation right now, and I don't want to mix it up on the short week," Nolan said Monday. "We seem to be making progress with our entire offense, so I want to keep that going. It's extremely important to me."
O'Sullivan started San Francisco's preseason opener Aug. 8 against Oakland and again Saturday during a 34-6 blowout of the Green Bay Packers. O'Sullivan had mixed results in those two games, but he came on strong against the Packers, guiding the 49ers on touchdown drives during his final two series in the second quarter.
O'Sullivan is 13-for-25 for 225 yards and hasn't been sacked during the preseason. He hooked up with rookie Josh Morgan on a 59-yard touchdown pass to spark the victory over Green Bay. But he also has thrown two interceptions and lost a fumble.
In the 10 drives directed by O'Sullivan this preseason, two resulted in touchdowns, three ended with turnovers and five ended with punts.
"He certainly has given me no reason to take him out right now," Nolan said. "We're staying consistent with our plan. I do feel confident we've got three (quarterbacks) who can win games. Two of them I know can because I've played with them in the past, and I'll continue to take a look at J.T., too."
O'Sullivan did not take any snaps in 11-on-11 team drills during the first seven days of training camp as Smith and Hill got all the work. But both players have come along slowly this summer in new coordinator Mike Martz's offensive system, and O'Sullivan has worked almost exclusively with the first unit the past two weeks. O'Sullivan, who was unavailable for comment Monday afternoon, worked in Martz's system last season when both were with the Detroit Lions. O'Sullivan played in four games with the Lions last season and threw the first regular-season passes of his NFL career.
Nolan had said throughout the first week of camp in July that he planned to evenly divide practice time and playing time during the preseason between his quarterbacks. But that plan has changed as O'Sullivan has evolved from long-shot status to top contender for the starting job.
Nolan said O'Sullivan will play most of the first half Thursday along with the rest of San Francisco's regulars. Smith will enter at quarterback after that, with Hill slated to see playing time late in the game.