FOXSports.com reported Friday that Oswalt, who remains a free agent, threw bullpen sessions for the Phillies and Red Sox last week.
Asked Wednesday if he knew Oswalt was throwing or what kind of shape he might be in, general manager Ruben Amaro Jr. said, "We think he's throwing. I guess he is."
Right-hander Vance Worley is on the disabled list with inflammation in his right elbow. The Phillies said they do not think it is serious, but if it is worse than they have said and he misses an extended period of time, they could sign Oswalt to rejoin the rotation.