STILLWATER, Okla. -- The Big 12's most promising contender for the College Football Playoff took a tumble in its toughest test to date, as West Virginia suffered its first defeat, 37-20 at Oklahoma State on Saturday.

After starting out 6-0 for the first time in a decade, the Mountaineers jumped into the top 10 while surprisingly becoming the buoy of the Big 12's playoff hopes.

But instead of West Virginia making a statement leading into next week's debut of the playoff selection committee's rankings, it was Oklahoma State that sent the message to the rest of the Big 12.

Mason Rudolph threw for three touchdowns to lead Oklahoma State's upset win over West Virginia on Saturday. Rob Ferguson/USA TODAY Sports

The Cowboys, now 4-1 in conference play, stamped their status as legitimate Big 12 title contenders with a dominating, well-rounded performance against what had been the league's most impressive team so far.

Oklahoma State quarterback Mason Rudolph was sensational, shredding the Mountaineers for 273 yards, three touchdown passes and a touchdown dash on third-and-goal, which gave the Pokes a 27-10 lead in the third quarter.

After a mild West Virginia rally in the fourth quarter, Rudolph came right back and produced a pair of clutch throws to Chris Lacy, including a 7-yard touchdown fade that put the Mountaineers away.

West Virginia's previously unheralded defense came into the weekend with plenty of hype after holding Texas Tech and TCU to 17 and 10 points, respectively, in blowout victories.

But Saturday, it was the Oklahoma State defense that delivered the game-changing plays.

Early in the second quarter, with West Virginia leading 10-6 and in control, Cowboys defensive tackle Vincent Taylor pushed his way into the Mountaineers' backfield and corralled quarterback Skyler Howard, who fumbled the ball while attempting to throw it away. The Cowboys scored two plays later to retake the lead for good.

In the third quarter, another big defensive play put West Virginia on the ropes. Cornerback Ramon Richards, who had previously dropped an uncontested interception, picked off Howard and weaved his way 59 yards to the West Virginia 9, setting up Rudolph's touchdown run.

With the Mountaineers going down, the Big 12's playoff hopes essentially hinged solely on Baylor, who also lost on Saturday. The Bears would have remained the league's lone undefeated had they beaten Texas. Baylor has an even shakier playoff case now with the loss, as they have netted only one quality victory this season -- at home over Oklahoma State.

Since that September loss, the Cowboys have bounced back to set themselves up for a November charge toward the Big 12 crown. Oklahoma State has only one ranked opponent left on the schedule: No. 16 Oklahoma in the regular-season finale.

The Mountaineers still remain alive in the Big 12 conference race, as well, with a key home tilt against the Sooners looming on Nov. 19.

But still without a victory over a ranked opponent, West Virginia's playoff hopes could be all but finished.