Oswalt left the team in order to be with former Houston reliever Ricky Stone, who suffered a medical emergency at his home after visiting with his former teammates in the visitor's clubhouse at Great American Ball Park on Thursday.
Oswalt didn't miss the entire game, however, as cameras showed him in the Astros' dugout during the fifth inning.
Stone, 33, a native of nearby Hamilton, Ohio, pitched for the Astros in 2002 and 2003. He also pitched for the Reds in 2007.
Alyson Footer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.