I would suspect the residuals of some sort of gall. Oaks are inordinately prone to a whole raft of galls that present in a myriad of different forms.

GG beat me to it!

My Q. macrocarpa reliably has several different forms of galls every season.

Some take more than one year to run their cycles.

My Q bicolors get very odd, large, woody ball-shaped galls composed of pressed together sections. Pic from internet.

Gee beng, that looks almost good enough to eat, like a 'Carmel Bite' or something.

So far, my immature bicolor hasn't produced any galls to date.

My bur oak has lots of these bullet galls and I've read that if you remove them before the insect bores its way out leaving a tiny hole in the husk and destroy them, it can reduce the number of galls in the future. Being my bur oak is still small, I did that this Fall so we'll see.