Unusual growth in oak leaf
These are likely cynipid wasp or midge galls. They are structures created by the mother insect inserting her eggs into the buds or very young leaf tissues. Usually, there are some fluids that mimic plant hormones and perhaps some viruses as well that hijack the tree's systems and cause it to create these structures for the larval insects to grow in.
Unless the tree is being defoliated repeatedly by these galls (they'll drop off in a month or so) it is unlikely the tree is suffering much. I suggest a wait and see approach.
There are treatments you can use, depending on tree size, if the galling becomes extreme and the tree begins to suffer. These treatments are themselves a bit hard on the tree or the general environment around it. Drenches with neonicotinid insecticides to be taken up by roots in the fall, trunk injections (very invasive), or foliar sprays (if the tree is small enough).