These appear to be bur oak leaf gall caused by the clustered midrib gall wasp (Andricus dimorphus). Galls are abnormal growths or swellings of plant tissue often caused by the attack of an insect. Galls on oaks are most often caused by small wasps or midges.  Sometimes these growths have an insect inside them, and sometimes they do not.  This particular species does have a larva/pupa inside.  When mature, they easily detach from the leaf.  The adult emerges from this growth in spring.

Most insect galls, including these, do not seriously affect the health of well-established trees. Occasionally, a heavy gall infestation causes severe leaf or stem deformities and early leaf drop. There is no way to "cure" the tree of galls once they appear. Leaf galls may not appear again the following season and they rarely require any sprays or other treatments.  Sprays are impractical and unnecessary since they don't cause long-term harm to the tree.

Here are a couple of resources that give you more information about galls.
https://extension.umn.edu/yard-and-garden-insects/insect-and-mite-galls

https://shop.iastate.edu/extension/home-garden/lawn-and-garden/pests-and-diseases/ent39.html

--Aaron