NORMAN, Okla. (AP) -- Oklahoma's play against South Florida didn't exactly build momentum for the start of the conference season.

The Sooners (No. 3 ESPN/USA Today, No. 2 AP) used a 21-point second quarter to take control Saturday night, but their 31-14 victory left coach Bob Stoops less than pleased.

"We've got an awful lot we need to get better at if we expect to compete in the Big 12,'' Stoops said. "We just didn't get anything going at all in the second half. Why? I don't want to hear any excuses why. Bottom line is, they stopped us.''

Nate Hybl threw two touchdown passes and Antonio Perkins returned a punt 82 yards for a score in the second quarter. That followed an opening period in which the Sooners (4-0) punted on their first four possessions.

Oklahoma, which was idle last week, scored again in the third quarter but netted just 30 yards on four possessions. In the fourth quarter, the Sooners had 17 yards on 14 plays.

The Sooners appeared headed for their third shutout of the year before South Florida (2-2) scored twice in the final 2 minutes against reserves.

"That is discouraging,'' Stoops said. "You get guys who get long faces in some other games that want to play, want to show they can play and that they can do what's necessary to be successful at this level, then they get in and get their opportunity and some really didn't perform very well.''

Perkins' second punt return for a touchdown this season got Oklahoma going in the second quarter. He wound up with 150 yards on six returns.

Hybl added his TD passes later in the quarter, each time after South Florida turnovers. The Bulls lost two of their three fumbles, had a pass intercepted and were penalized 15 times for 124 yards.

"Turnovers throughout the game were critical,'' coach Jim Leavitt said. "Oklahoma lives off turnovers. You can't do those things and expect to win.''

The punt return by Perkins, who also caused a fumble and made an interception, came after South Florida had a trick-play touchdown wiped out by penalty. On the play, quarterback Marquel Blackwell threw to his left to running back Brian Fisher, who then threw downfield to a wide-open DeJuan Green. But Blackwell's throw to Fisher was ruled a forward pass.

Hybl, who was 19-of-30 for 177 yards, threw a 3-yard scoring pass to fullback J.D. Runnels to cap an eight-play drive. The drive began after a South Florida fumble and was kept alive by a 14-yard completion to tight end Trent Smith on a third-and-13 play.

The Bulls then drove to the Oklahoma 16-yard line before fumbling the ball away again. Passing on every down, Hybl had a 25-yard completion to Smith on third-and-2 to set up a 23-yard TD pass to Will Peoples, who made a nice diving catch.

Peoples had five catches for 41 yards, all in the first half. He entered the game with four receptions on the year.

Oklahoma's only second-half touchdown drive began at the South Florida 45 after Perkins' interception. The drive was helped by two Bulls penalties and ended with Kejuan Jones scoring on a 1-yard run.

The South Florida touchdowns came on a 2-yard pass from Ronnie Banks to Mark Feldman with 1:59 left and a 29-yarder from Banks to Ryan Hearn with 5 seconds left.