Case McCoy will make his third start this season in the Red River Rivalry game at Dallas.
Ash has missed 2½ games this season while recovering from a concussion suffered Sept. 7. He returned to play in the first half of Texas' 31-21 win over Kansas State two weeks ago but was pulled from the game at halftime due to symptoms related to his concussion.
In the 10 quarters he has played this season, Ash has passed for 760 yards and seven touchdowns with two interceptions.
Ash did not travel to Texas' 31-30 win against Iowa State on Thursday. In his place, McCoy threw for 244 yards and a touchdown on 26-of-45 passing and rallied the Longhorns with the third fourth-quarter comeback of his career.
Texas coach Mack Brown hasn't disclosed what setback Ash suffered during halftime of the Kansas State win. But Brown has said he's leaving the decision on Ash's status to team doctors. The Oklahoma game will be 21 days since Ash's last start for the Longhorns.
Last year against Oklahoma, McCoy relieved an injured Ash in the fourth quarter of Texas' 63-21 loss and threw for 102 yards and two touchdowns.