Oak gall is a growth that forms on the leaves, stems, or roots of oak trees when certain insects, like wasps, lay their eggs in the plant tissue. The tree reacts to the presence of the eggs by producing a mass of plant tissue, which creates a protective environment for the developing larvae. These galls can vary in size and shape, often appearing as round or irregularly shaped bumps. While they may look unsightly, oak galls are generally harmless to the tree and can even provide habitat for various organisms. They are also used in traditional medicine and as a source of tannins.