The Oak Apple, actually known as an Oak Apple Gall is caused by a tiny gall wasp, Biorhiza pallida. It can be found between May and June on oak twigs where the female has laid her eggs in the leaf bud. Inside the gall there are a number of chambers, each housing a larva which eats its way out. Adults emerge in June and July.
My hubby spotted this oak apple on a walk through the woods, a lucky find among all the autumn leaves. I found it quite fascinating how the gall is absolutely super-glued to the oak leaf. Photographing it was great fun.