These are galls, which are produced by Gall Wasps. Each species of Gall Wasp lays its eggs in a specific part (leaves, branches, bark, etc.) of a specific kind of plant, and 70% of all Gall Wasps lay their eggs in oak trees. Scientists are still not certain exactly what triggers the plant to form a gall. It may be a chemical that the wasp injects, or it may be a virus that the wasp carries. Whatever the cause, the plant tissue grows into a large, mass. The egg hatches, and the larval wasp eats the inside. Once it matures, it eats a hole in the gall and flies away.