Reliever Scott Mathieson was recalled from Triple-A Lehigh Valley to replace Oswalt, who was sidelined with a back injury earlier this year.
Oswalt is 4-6 with a 3.79 ERA in 13 games. The three-time All-Star is 1-5 with a 4.06 ERA over his last eight starts.
In other moves, catcher Brian Schneider was activated from the disabled list and lefty J.C. Romero was given his unconditional release. Catcher Dane Sardinha was outrighted to Lehigh Valley to make room for Schneider.
