SANTA CLARA, Calif. — J.T. O'Sullivan isn't getting ahead of himself, even though the sixth-year veteran is starting for the third consecutive week at quarterback for the San Francisco 49ers.

O'Sullivan has assumed command in the three-way competition with veteran holdovers Alex Smith and Shaun Hill to become San Francisco's starter for the Sept. 7 regular-season opener against Arizona. But for now, he insists he's only focused on Thursday night's exhibition game against the Chicago Bears.

"Everybody gets excited during camp for the regular season, but everybody knows this third game is a big game in the preseason," O'Sullivan said Tuesday.

Coach Mike Nolan says the team still has not decided which quarterback will start the opener, but actions are speaking louder than words at the 49ers training camp. O'Sullivan has taken virtually every snap with the first-unit offense since Aug. 6, when he emerged as the leading contender at the position.

Most NFL teams typically consider the third exhibition game a dress rehearsal for the regular season, a time when starters see extensive action before getting a rest in the preseason finale. If O'Sullivan hasn't won the starting job yet, he couldn't be any closer to it than he is now.

The San Francisco offense, which ranked dead last in the 2007 NFL rankings in several major statistical categories, was at its best during Saturday's 34-6 rout of the Green Bay Packers.

The 49ers had been slow to absorb new offensive coordinator Mike Martz's system during summer practice sessions, and that carried over to the team's 18-6 loss to Oakland in San Francisco's Aug. 8 exhibition opener. But with O'Sullivan at the controls, the offense began to click against Green Bay. O'Sullivan passed for 154 yards in two quarters of action and directed two touchdowns drives.

O'Sullivan played in Martz's offense last year when both men were with the Detroit Lions. His knowledge of the system and comfort in it has given him an advantage this summer.

But O'Sullivan also has added excitement to a San Francisco offense that averaged just 237 yards per game last season — almost 40 fewer than any other NFL team — and was last in the league with 219 points scored.

"The leadership and qualities he has just makes you kind of want to latch onto it and follow a guy like that," 49ers receiver Josh Morgan said. "J.T. knows the offense and he gets in there and takes command of the huddle. He goes out there and makes plays and gives us his all every time, and that's what you want in a quarterback."

Morgan, a rookie who leads the NFL in exhibition play with five receptions of 20 yards or longer, has been O'Sullivan's favorite target so far during the preseason with several of San Francisco's veteran receivers out with injuries. The pair hooked up for a 59-yard touchdown reception against Green Bay.

O'Sullivan is in the midst of his longest stint as a No. 1 quarterback during a career that has seen him play with seven other NFL teams. A sixth-round draft pick of the New Orleans Saints in 2002, O'Sullivan has thrown just 26 regular-season passes all coming last year with the Lions.

The starting position he has been chasing throughout a journeyman career finally appears within reach. But O'Sullivan knows he must continue to keep a tight grip.

"I'm not satisfied to be the starter coming into the third preseason game," O'Sullivan said. "Everybody wants to play and be the guy. So we'll go from there, but this is another opportunity to play well this week."

That's all it figures to take Thursday from O'Sullivan to get the official starting nod over Smith, the No. 1 overall selection of the 2005 NFL draft. Smith took every offensive snap at quarterback for the 49ers during a promising 2006 season, but his career took a detour last year when he suffered a separated throwing shoulder in Week 4 that required season-ending surgery.

Smith has followed O'Sullivan into the first two preseason games and will do so again Thursday. Smith said he's still competing for the starting position with two preseason games remaining.

"They haven't said otherwise," Smith said. "I'm going out there treating it like that. You have to. I'm not sure what they're thinking or what is going on upstairs, but I'll get my reps and I'll be ready for them. I'm trying to take advantage of the things I can control."