” The little round speckled balls that you see on the ground and on the oak leaves are called “oak galls.” Inside of the oak galls are insects called gall wasps. In the spring before the oak leaves are fully grown, a wasp will lay an egg on a leaf. The egg causes a chemical reaction that makes the tree grow a case around it. This case protects the egg and larvae from being eaten by birds. Then the larvae eat out the inside of the gall as they grow. If the oak galls have tiny holes in them, that means the wasps inside have already emerged.
Oak galls can sometimes grow to be the size of a ping pong ball but their average size is three to four centimeters in diameter. ”
From Oak Island Nature Trail Guide: George Middle School and ODFW