Houston optioned right-hander Runelvys Hernandez to Triple-A Round Rock.
Oswalt (7-8, 4.56 ERA) had not started since July 11 at Washington, when he re-aggravated a hip strain and left after one inning. He originally hurt himself June 30 against the Los Angeles Dodgers.
Oswalt went on the DL on July 19, after an MRI exam confirmed the strained left hip abductor and revealed a herniated disc in his back.
The 30-year-old Hernandez was recalled when Oswalt went on the DL. He made four starts with the Astros, going 0-3 with an 8.38 ERA in 19 1/3 innings.