Hi Bill,
Rather than acorns, those are acorn plum galls. A gall is formed when a non-stinging wasp lays an egg on, in this case, the tissue from which the acorn grows, and along with the egg the wasp injects a substance that causes the tree tissue to grow around the egg. If you cut open one of the galls, you'll find one larvae, likely suspended within the gall. Below is a link to an article that explains the issue in more detail. 
https://www.canr.msu.edu/news/tree_galls_are_rarely_cause_for_concern
These galls will not harm the tree or alter acorn production. Some years there are more galls than others. No reason for concern or action. Enjoy the wonders of nature!
Thank you,
-Julie

Thank you Julie. Very interesting. Same method as ball galls on goldenrod, etc

Yes, same as the goldenrod galls. Maple leaves often get leaf galls, too, in the form of little red threads arising from leaves. 

It's fascinating!
Take care.