Those look to me like insect galls. There are several species of insects that lay their eggs on tip or flower buds of oaks (yes, oaks have flowers). As the insect develops it's waste acts like a growth hormone on the plant forcing extremely rapid cell growth and division in the epidermis. Since the growth and divisions are equal in all directions with respect to the maggot's waste, the gall is usually spherical. Mature galls have a tough epidermis, abundant aerenchyma that soon degrades leaving open space with scattered radially disposed vascular bundles attached to the epidermis and to the encysted maggot located in the center of the gall.

Just a guess.