Chelf passed for a career-high 370 yards and accounted for four touchdowns as No. 11 Oklahoma State took down another national-championship hopeful, easily defeating No. 3 Baylor 49-17 on Saturday night.
The loss for the Bears leaves Alabama, Florida State and Ohio State as the lone remaining unbeatens near the top of the BCS standings - and they'll certainly line up in that order when the new numbers come out Sunday.
Oklahoma State has now won 10 or more games in three of the last four seasons, and can win the Big 12 by beating rival Oklahoma at home in two weeks.
Chelf, who lost the starting job earlier this season for the Cowboys (10-1, 7-1 Big 12 Conference), threw for three touchdown passes and ran for a score - continuing his mastery of the stunned Bears in front of a record crowd of 60,218 in Boone Pickens Stadium.
The senior also had a 48-yard reception and finished with 438 yards of total offense. He was 19-of-25 passing. Tracy Moore added five catches for 126 yards for Oklahoma State.
Bryce Petty was 28-of-48 passing for 359 yards for Baylor (9-1, 6-1), which had its 13-game winning streak snapped and hasn't won in Stillwater since 1939.
The Bears entered Saturday averaging a national-best 61.2 points and 684.8 yards per game.
They had no answers for a Cowboys' defense that held the high-powered attack in check most of the way.
Oklahoma State forced three Baylor fumbles and also stopped the Bears on two key fourth-down attempts in the second half - both which led to Cowboys' touchdowns.
Baylor finished with 453 yards of total offense, and Antwan Goodley had 10 catches for 118 yards receiving.
The Cowboys, led by Chelf, finished with 594 yards of total offense.
After falling behind 35-3, Baylor appeared on the verge of cutting the lead to 35-17 early in the fourth quarter.
However, Petty fumbled deep in Oklahoma State territory and Tyler Patmon returned it 78 yards for a touchdown to put the Cowboys up 42-10.
The score put the exclamation point on a thoroughly one-sided performance.
Chelf lost the starting job earlier this year to sophomore J.W. Walsh, slowly working his way back to the role of top option for the Cowboys. The senior accounted for four touchdowns in last week's win over Texas, but he was at his absolute best on Saturday night - capping the stunning performance with a 33-yard touchdown pass to Jhajuan Seales.
Baylor appeared on its way to the early lead when Petty broke free on a third-and-two on the Bears' second possession - with no defenders in his path to the end zone.
However, the quarterback - who entered the game leading the Big 12 in total offense - tripped short of the end zone and fell down at the Oklahoma State 1-yard line. Two plays later, Shock Linwood fumbled when he tried to stretch out for the touchdown - with James Castleman knocking the ball loose and recovering the ball for the Cowboys.
That was followed by a 99-yard touchdown drive for Oklahoma State. Kye Staley capped the quick turn of momentum with a 2-yard touchdown run top put the Cowboys up 7-0, a drive that included a 51-yard pass from Chelf to Marcell Ateman.
Oklahoma State pushed its lead to 14-0 in the second quarter when Chelf found Charlie Moore for a 12-yard touchdown over the middle. The completion was Chelf's 11th-straight of the game.
Chelf didn't have an incompletion until Oklahoma State's final possession of the first half, and he finished the half 12-of-13 passing for 238 yards.
The Cowboys held Baylor to three three-and-outs on seven possessions in the first half. The Bears entered the game having been held to 17 three-and-outs all season, and their three points at halftime was their lowest first-half total since being held scoreless by Oklahoma State in 2011.
Chelf has now thrown for 703 yards in his last two games against Baylor, dating back to last season's 41-34 win for the Bears over the Cowboys.