O'Sullivan clinched the job with a strong performance during Thursday night's 37-30 exhibition victory over Chicago, and 49ers coach Mike Nolan said it was time to make O'Sullivan's status official after a summer of uncertainty.

"J.T. O'Sullivan will be our starting quarterback going into the season and I'm expecting him to succeed," Nolan said Friday. "I'm very confident J.T. is at the point where he's our best man for the job right now."

O'Sullivan, a long shot to become the starter when training camp began at the end of July, overtook veteran holdovers Alex Smith and Shaun Hill during the past two weeks. The sixth-year player, now with his eighth NFL team, has never started a regular-season game in the league.

That is about to change.

O'Sullivan was picked to start San Francisco's preseason opener Aug. 8 against Oakland and has been practicing and playing with the first unit since. He didn't 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 struggled to grasp new offensive coordinator Mike Martz's complex system, and O'Sullivan was given a chance in the lead role earlier this month.

O'Sullivan worked in Martz's system last year when both were with the Detroit Lions. He played in four games with the Lions last season and threw the first regular-season passes of his NFL career.

He moved the offense during San Francisco's first two preseason games and then had a stellar performance during Thursday's win in Chicago. He completed 7 of 8 passes for 126 yards and guided San Francisco on scoring drives each of his three series.

O'Sullivan left the game during the first minute of the second quarter after completing a 37-yard touchdown pass to Jason Hill that gave San Francisco a 13-10 lead.

"This hasn't been a case of Alex and Shaun not performing well," Nolan said. "This has been a case of J.T. performing very well. That's why we are where we are. J.T. has very good command of the offense, he's played well in all three preseason games, and now it's time for us to get ready for the regular season."

Nolan said Smith, the team's regular starter the past two seasons, would be O'Sullivan's backup entering the season while Hill would assume the role as the team's No. 3 quarterback.

The 49ers invested $24 million in guaranteed money in Smith as part of a $49.5 million deal when they made him the No. 1 overall selection in the 2005 draft. They also signed Hill to a $6 million contract this year after he went 2-0 as a starter last December following injuries to Smith and backup Trent Dilfer.

O'Sullivan signed a one-year deal for the veteran's minimum salary of $645,000, but what matters more for the 49ers than those numbers is his performance on the field.

"We want to play our best players that give us the best chance to win," Nolan said. "For our football team's sake, it's time to get ready for our opening game. This (decision) puts more focus on where it should be now."