I'm pretty sure this is a California Gall Wasp (Andricus quercuscalifornicus) but am looking for confirmation. My ID is based on the gall. After years of seeing these galls, I found one that was "fresh" and still intact -- no exit holes. I took it home and put it in a jar for a few days. Magic! This picture shows the exit hole along with the little wasp that emerged. I'm uploading a separate picture of the gall, too, since you can't see much here.