It's an 'oak apple', a type of gall.

from our friends at moo U from the link below:

Galls are one of the more intriguing sides of nature to view and behold. They come in all shapes, sizes, colors and textures – truly one of nature’s more “artistic” flourishes! Oak trees, in particular, appear to get many different types of galls.

https://www.canr.msu.edu/news/tree_galls_are_rarely_cause_for_concern

besides.. what are you going to do about it on a huge oak tree ... as compared to yours ... the tree didnt get huge over a hundred years.. if something like this was a problem ...

ken

Glad to see this posted. I just found 2 of those on my oak tree last week. Had never seen them before.

Thanks guys! I had never seen this either. Gracie, are yours on a full-grown oak?

:)
Dee