Question
Is there any harm in my dog eating Oak Leaf Galls? He eats the acorns too!

Answer
Galls are abnormal plant growth or swellings comprised of plant tissue. Galls are usually found on foliage or twigs. These unusual deformities are caused by plant growth-regulating chemicals or stimuli produced by an insect or other arthropod pest species. The chemicals produced by these causal organisms interfere with normal plant cell growth.

I do not know of any effects on dogs if eaten. Some do contain mites and if eaten could bite the stomach BUT with the acid in stomachs the mite would not live very long .  Sorry I can not find any reference to problems with animals eating the galls.

I've seen oak leaves and acorns on a few "toxic-to-dogs" lists, but most sources say that there is no serious risk in moderation. They talk about the possibility of blockage if swallowed whole (not easy to digest) and stomach upset if eaten in quantity because of the tanic acid. It will depend on the size and age of the dog.