What are the purple berries in oak trees?

The purple berry-like growths that appear in oak trees aren't berries at all. They're oak galls, which the tree develops in response to being infested by certain kinds of small wasps.

The galls appear on leaves, twigs and branches after the wasps lay their eggs on the oaks. Wasp larvae feed on the material inside the galls. Leaf galls pose little health risk to a tree, but twig or branch galls can kill the tree if the infestation is heavy. Pesticides are not recommended to control galls. Destroying infected fallen leaves and pruning twigs and branches when small galls are noticed may be helpful.