While my husband doesn't share my compulsion to name every natural thing he sees, he cheerfully humors me and brought this gall home from California. He found it in the street near a restaurant. Looking at the tree on google maps, it doesn't look like an oak, but there's no telling what tree it came from since it was in the street next to the curb. There appeared to be at least one exit hole. Andricus quercuscalifornicus?