Eric Striker ended the game by recovering a fumble for a touchdown for Oklahoma, which has won at least 10 games in four consecutive seasons.
Sooners coach Bob Stoops improved to 8-1 in his career against Cowboys coach Mike Gundy.
Desmond Roland led No. 6 Oklahoma State (10-2, 7-2) with 144 yards rushing and accounted for three touchdowns.
The Cowboys, who were trying to win their second Big 12 title in three seasons, appeared to have their spot in a BCS bowl game secured when Roland scored on a 1-yard touchdown run with 1:46 remaining to put Oklahoma State up 24-20.
The score capped a seven-play, 89-yard drive for the Cowboys, who led for the entire game until Oklahoma went up 20-17 early in the fourth quarter. Oklahoma State quarterback Clint Chelf completed five straight passes for first downs on the drive, and it appeared the Cowboys had the Big 12 title in hand.
No. 9 Baylor 30, No. 23 Texas 10 • In Waco, Texas, Bryce Petty and the Bears closed out of their old stadium with quite a fiesta.
In what became a de facto Big 12 championship game after Oklahoma State's loss, Petty threw for 287 yards with touchdown passes on the first drives after halftime, and the Bears won their first title in the league by knocking off the Longhorns (8-4, 7-2 Big 12).
The Bears (11-1, 8-1), who never even had a winning record in the Big 12 before coach Art Briles arrived six years ago, have the first 11-win season in school history and are headed to the Fiesta Bowl. That is the reward for their first outright title in any league since winning the Southwest Conference in 1980 when Mike Singletary called Floyd Casey Stadium home.
No. 15 UCF 17, SMU 13 • In Dallas, Blake Bortles threw for 242 yards and ran for two touchdowns as No. 15 UCF celebrated a BCS bid already in hand by rallying to beat SMU in front of just a few hundred fans who braved a brutal ice storm.
With the school's first BCS berth secured by Louisville's win at Cincinnati on Thursday, the Knights were sluggish before recovering for a school-record eighth straight win and the outright title in the first year of the American Athletic Conference.
UCF (11-1, 8-0 AAC) has both of the 34-year-old program's 11-win seasons in the past four years. The Knights went 11-3 in 2010 and beat Georgia in the Liberty Bowl.
SMU (5-7, 4-4) saw its record bowl streak end at four seasons in the coldest home game in school history 24 degrees at kickoff.
It matched the 1983 Sun Bowl in El Paso as SMU's coldest game. It was UCF's coldest game by far.
No. 18 Oklahoma 33, No. 6 Oklahoma 24
• Cowboys' BCS hopes dashed.