The Rangers signed Oswalt on May 29 to a Minor League contract after Neftali Feliz went on the disabled list with a sprained right elbow. The Rangers also put Derek Holland on the disabled list this week.
"Obviously with the two injuries, there is a temptation to accelerate Roy's program and go get him," general manager Jon Daniels said. "We want to make sure he's ready to come in and win at the Major League level."
Oswalt went four innings Thursay night at Omaha and allowed five runs in four innings. He gave up six hits, walked one and struck out four while throwing 56 pitches.
"He had one rough inning," Daniels said. "Overall he got his work in. The main thing is to try to extend his pitch count and get into game shape."
Oswalt will likely throw approximately 80 pitches in his next start against Albuquerque. After that, the Rangers will re-evaluate his situation.