Hill won the first two starts of his career late last season after spending his first five NFL campaigns as a seldom-used backup or third-stringer.
He lost a three-way competition with O'Sullivan and Alex Smith during training camp this summer, but O'Sullivan has struggled with 11 interceptions and 11 fumbles in seven and a-half games.
Hill replaced O'Sullivan for the final play in the first half of the Niners' 34-13 loss to Seattle yesterday.
O'Sullivan fumbled on the 49ers' first two possessions and later threw an interception that was returned 75 yards for a score.
Hill played the entire second half, going 15-of-23 for 173 yards and a touchdown.