It is one of the woolly oak galls. For whatever reason, oaks are inordinately prone to a wide number of galls, which by and large tend to be harmless. Galls are an example of commensalism, a relationship in which one organism benefits while the other is neither harmed nor helped.

Pruning doesn't really hurt but it doesn't help anything either.

I get woody galls on my swamp white oak that don't seem to cause much problems. Not my pic, but a search showed what they look like.

It looks like your oak has grown a pair of galls.

Commensalism. That’s a new term for me. Thanks GG!

Isn't that a great word?? I first encountered it when studying lichens, although those are more of a case of mutualism rather than commensalism. (And don't ask me to explain the differences :-))

Do twig galls cause gouty oak galls?