It's a leaf gall of some type. I read somewhere that oaks have more different gall issues than just about any other genera. But they almost always tend to be merely cosmetic in nature and of no real harm to the tree.

I am a bit surprised that any qualified arborist wouldn't recognize a leaf gall........they are very common and occur on many different types of trees.

I am a bit surprised that any qualified arborist wouldn't recognize a leaf gall

Yes - does this party market themselves as a certified professional? One with credentials that can be verified? Or when it gets down to it is this really just another guy with a truck and some saws etc.? Who's in it primarily for the money that can be made from topping, removals and the like? Jive talkers who sell themselves as "arborists" but are actually tree killers looking for big bucks quick are out there. The attraction is that because it's often hard and dangerous work they can make sometimes thousands a day wrecking trees.

perhaps he isnt sure.. and going to send his brother the tree sprayer over.. for some six hundred dollar annual sprayings for the most likely benign galls ...

you could probably take a couple leaves to your COUNTY extension office and they can ID them for you ...

ken

ps: add you location to the search if you wish

https://duckduckgo.com/?q=isa+arborists&t=ffcm&ia=web

https://bugguide.net/node/view/282099/bgimage

This look like your fellows? Just a stab in the dark.

Sigh...a large percentage of so called arborists in our area declare themselves as such b/c they read a manual. You can also have company owner be a true arborist but he can hire field people who are not. Many unsuspecting clients think that they are, just b/c they found co. while doing an arborist search and most are too shy to ask directly about actual creds. Field people get away with it. It is really a terrible situation.