It’s an oak apple, but I thought oaks have acorns. They are called acorn trees not apple trees. So what’s an apple doing growing on an oak? Some weird cross pollination?
It’s a gall. A little wasp nursery. Momma wasp, along with some helper mites, directed an oak tree to make this nifty house for one of her kids. One larva per gall. Plants and animals teaming up.
Inter-kingdom cooperation.