O'Sullivan didn't get that opportunity often during training camp this summer. He didn't take one snap in 11-on-11 team drills during the first seven days of camp in July, and top veteran receivers Bryant Johnson and Arnaz Battle have been out most of the past two weeks with injuries.

Johnson is projected to start at split end this year with Battle penciled in as the team's No.3 wideout. The other starter is 15th-year veteran Isaac Bruce, who has been limited in both practice and exhibition games throughout the summer because the team wants to keep the 35-year-old fresh for the regular season.

Battle returned to individual practice drills Saturday after missing the past two exhibition games with a hamstring strain. Johnson has yet to play during the preseason and is expected to miss Friday's exhibition finale against San Diego because of a similar injury.

The 49ers also have been without veteran Ashley Lelie (calf strain) most of the summer, but O'Sullivan isn't concerned about getting up to speed with the wideouts before San Francisco's Sept. 7 opener against Arizona.

"I expect everyone who plays to know exactly what to do and be exactly where they're supposed to be when they're supposed to be there," O'Sullivan said Saturday. "I think they expect the same from me also. I'm comfortable with the system and those guys doing their job. We do those things and we'll be fine."

O'Sullivan won the starting job during a summer competition with veteran holdovers Alex Smith and Shaun Hill because of his comfort and precision in new offensive coordinator Mike Martz's detailed system. O'Sullivan learned that system last year when both he and Martz were with the Detroit Lions.

O'Sullivan has produced a solid preseason thus far even though he has been throwing primarily to young receivers, some of them second- and even third-teamers. His favorite target during an Aug. 16 victory over Green Bay was rookie sixth-round draft pick Josh Morgan, who had a game-high five receptions that night and hooked up with O'Sullivan on a 59-yard touchdown play.

Second-year receiver Jason Hill, a third-round pick in 2007, was O'Sullivan's top target during Thursdays wild 37-30 victory at Chicago. O'Sullivan was impressive during his quarter of action, completing 7 of 8 passes for 126 yards and leading San Francisco on scoring drives each of his three series in the game. His only incompletion came on a botched shovel pass.

O'Sullivan departed the game after scrambling to complete a 37-yard touchdown strike to Hill in the corner of the end zone. That left O'Sullivan 20 of 33 for 351 yards passing with a quarterback rating of 91.8 this preseason.

"That was a hell of a play by J.T. scrambling out and taking a hit," Hill said. "I've got a lot of confidence in him. He comes in the huddle and he commands our attention. It means a lot that (O'Sullivan's the starter), just because it's good for team morale and we can rally around J.T. now and let him know we have the confidence in him to get the job done."

Coach Mike Nolan is confident O'Sullivan can hit the right spots regardless of which receivers the 49ers have in the game. Battle might be ready to play by Friday.

But when the regular season begins, Nolan said, "It looks like we'll have to play our wide receivers a little bit as far as rotating because of the injury situation we had in training camp and that a number of them still have to get the detail of their jobs down.

But whoever's in the game with (O'Sullivan), Nolan said, "I expect some things. That's what's real important about our offense it's a disciplined attack. You have to be at the right spot. Whoever's in there at receiver has to be accountable and trustworthy, or otherwise the quarterback gets fooled."