I found these oak galls growing on a small oak tree that is part of a hedgerow we planted a couple of years ago. It may look like something you could eat – one species is known as ‘oak apple’, but it is in fact the larva stage of a gall wasp.
Our native oaks are hosts plants to up to 30 different species of gall wasps. Depending on the type of gall wasp, the female inserts eggs into either a vegetative bub, a flower bud, an acorn or even the root of the tree. When it hatches the grubs secrete a chemical that results in this abnormal growth, know as a gall. It is this that encloses the larva of the wasp. The gall does not harm the tree.
I think these ones are Oak Marble galls (Andricus kollari), also known as oat nut, but am open to correction.