[Eric in the Ozarks]

It’s a collection of galls.

Mites or small wasps deposit eggs in the leaves of trees or vines. The “deposit” contains food for the egg.

The leaf reacts to these deposits by walling off the eggs with these “bumps.”

Brown bumps on oak leaves are more common. This is the first time I’ve seen little red horns like this.

[JRandomFreeper]

Weird. I've got oaks around, and collect the galls (golf ball sized) to make ink (along with rust). But I've never seen anything like that.

Thanks for the explanation.

/johnny