Oak club galls form on several oak species when oak club wasps (Andricus coortus) lay their eggs at the tip of new stems in the spring. The oak club is a cynipid wasp. Adult club gall wasps emerge the following spring.
The oak club gall is polythalamous (containing several larval chambers). Stunted leaves and viable buds often protrude from the globular gall.
These oak club galls were on Oregon white oaks (Quercus garryana) along the trail originating at the Baum Lake spillway (Shasta County CA). Although not visible in the photograph, there were live larvae in the galls.