Chelf, a senior who began the season at starter but was replaced by J.W. Walsh, entered the game against TCU last weekend and helped lead the Cowboys to a 24-10 win.
Chelf has completed 54.5 percent of his passes with three touchdowns and one interception in five appearances.
Walsh is Oklahoma State's leading rusher but has five interceptions along with nine touchdowns.
No. 19 Oklahoma State is 5-1 overall, 2-1 in the Big 12.