"The determining factor was that J.T. had the better command of the three," Nolan said. "This is not a case of Shaun or Alex not playing well. This is a case of O'Sullivan performing very well."
Smith will be the backup and Hill the No. 3 quarterback.
O'Sullivan passed for 126 yards and a touchdown in the 49ers' 37-30 preseason win over the Chicago Bears Thursday.