There are golf ball-sized growths with horn-like projections on the branches of my pin oak. What are they?

FAQ
Question: 

There are golf ball-sized growths with horn-like projections on the branches of my pin oak. What are they?

Answer: 

The growths on the pin oak are a type of gall.  Galls are abnormal growths of plant tissue induced to form by mites, insects, or other small organisms.  The galls on the pin oak are called horned oak galls because of the horn-like projections that protrude from the surface of mature galls.  Horned oak galls are caused by a tiny wasp (Callirhytis cornigera).  

The life cycle of Callirhytis cornigera is unique.  It consists of alternating generations that give rise to two distinct types of galls produced by two groups of female wasps.  In spring, wasps emerge from mature horned oak galls.  At this time, all of the wasps are female.  The female wasps deposit their eggs on developing leaves.  This initiates the development of tiny leaf galls along the leaf veins.  Adult wasps emerge from the leaf galls in mid-summer.  This group of wasps includes both male and female wasps.  Mated females lay their eggs on twigs, initiating the development of horned stem galls.  Each gall houses anywhere from 1 to 160 larvae.  Each larva is housed individually in a cone-like structure within the gall.  Wasp development in twig galls takes almost 3 years.  As the immature wasps reach pupation, the horns rise from within the gall to break through the surface and release the adult wasps.