Chelf also threw two scoring passes as Oklahoma State (8-1, 6-1 Big 12) won in Austin for the third straight meeting. This one may re-ignite pressure on Texas coach Mack Brown, who was dealt his worst home loss in 16 seasons - and at a time when speculation about his job security had just begun quieting down.
A resurgent Texas had won six in a row and this week returned to the Top 25 after a two-month absence. Now the Longhorns (7-3, 6-1) are almost certain to fall unranked again, but more costly in this blowout was relinquishing control of their destiny in the Big 12.
Texas and Oklahoma State still have yet to face undefeated No. 4 Baylor, which was playing Texas Tech later Saturday. The Cowboys host the Bears next week.
Chelf threw for 197 yards and an interception. He was also Oklahoma State's leading rusher, running for 95 yards that were largely piled up with long sprints from the shotgun that cut through the middle of the Texas defense.
Texas running back Malcolm Brown ran for 79 yards and a touchdown.
A close first half turned into a runaway behind two near-misses by Texas in a span of less than a minute. First up was Texas safety Adrian Phillips missing a would-be interception in the end zone, with the ball ricocheting off his hands and into the arms of Tracy Moore for an Oklahoma State touchdown.
Even more deflating for Texas was the ensuing Texas drive, when Cowboys safety Justin Gilbert picked off Chase McCoy just 18 seconds before halftime and returned it 43 yards for another score.
McCoy finished 26 for 39 for 221 yards and would throw two more interceptions. Linebacker Caleb Lavey picked off one of them, running it back to Texas' 21 to set up a Chaff touchdown pass to Charlie Moore on the very next play.
Charlie Moore led all Oklahoma State receivers with six catches for 83 yards.
Oklahoma State played without leading wide receiver Josh Stewart, who didn't make the trip after appearing to injure his left leg in a win over Kansas last week. Stewart is also the nation's leading punt returner, but aside from understudy David Glidden bobbling a few catches, the Cowboys hardly noticed his absence.
On hand to watch Brown's worst loss ever at Royal Memorial Stadium was new athletic director Steve Patterson, who was hired from Arizona State earlier this month to replace DeLoss Dodds. Patterson has said he doesn't intend to "make change just for the sake of making changes" as he takes over the nation's wealthiest athletic department.
Brown began the season talking about Texas being ready to compete for a national title again after a run of uneven seasons since losing to Alabama in the title game in 2009. But the Longhorns instead started out by losing badly to BYU on the road and getting blown out at home by Ole Miss.