oak leaf gall
I have these brown sticky BB-sized balls on the underside of my burr oak leaves. Is that some type of gall, and what do I need to do about it now or possibly in the spring? Is it harmful to the tree? There are wasps all over the tree. The infestation seems to be pretty heavy.
You are correct that you have a type of gall, specifically Oak Leaf Gall, on your Burr Oak. Below is a link to a CSU Fact Sheet on insect and mite galls. Please note that it says galls rarely do any real damage to plants and that essentially all insect galls found on oak are produced by gall wasps. That would explain all the wasps on your tree. No treatment is recommended.
Boulder County Master Gradener