They are galls of some sort. Can be caused by all sorts of things, often insects. Not harmful to the tree.

imo .. leaf damage on deciduous trees is limited to the season they occur.. and then they fall off .. problem solved ...

in the extreme ... even if a tree is completely denuded of leaves .. the problem usually goes away .. once the leaves are gone .. if for no other reason.. they have nothing left to eat ... and an otherwise fully established healthy tree can releaf ...

knowledge is power ... and you learned something to day .. but.. nothing to worry about. nor take action on ...

ken

ps: keep in mind.. 99.9% of that leaf is unaffected .. so its still doing its job creating energy from sunshine .. so basically no impact at all.. on the tree ...

Heavy consistent infestations can be harmful but atypically so usually nothing to worry about.

I did learn something new today, so that's good. Never knew what gall looked like, now I get to worry about oak mites! Anyone know if it bother dogs? I planned on redoing my fence which the new area has an oak tree in, but I will change my plan if they are an issues for dogs; Have/had a plan in my head, ordered the fencing and most of it's here, but I can change the plan to leave out the oak area, but will be disappointed.

I live where Oaks are plentiful and widely distributed. I've never heard of insects from trees harming animals. The only concern about Oak trees is some have an issue with acorns dropping and with a large tree can accumulate in quantities (certain years). My riding lawn mower made quick work of acorns although it made quite a racket (noisy) while mowing.I wore head sets anyway so nbd.

Oaks are more of a legacy tree and will be around for a while.

No problem with dogs (my Quercus lobata gets galls that eventually get to the size of tennis balls and the dogs play with them). And not necessarily caused by mites. I've never seen a Q. lobata without these galls; they are an established symbiotic relationship.

Here is one of the dogs with a ball and some galls! And a good article about oak galls.

KQED: The crazy cribs of parasitic wasps

Cute! Looks like a great companion.