After Trevor Knight suffered an injury, Thomas started three games at quarterback last season for the Sooners. He completed 46 percent of his passes for 342 yards, two touchdowns and four interceptions. He also ran for 141 yards.
Thomas, listed as an outfielder on the team's 2013-14 roster, appeared in 14 baseball games last year.
He is expected to battle Knight and Baker Mayfield, who transferred in from Texas Tech last year, for the starting job.