Oak apples are an insect home called a gall. The “apple” is actually a mutated oak leaf. The strange growth pattern is caused by an insect egg, from a kind of wasp that lays its egg in a freshly-emerged oak leaf in spring. A chemical produced by the egg causes the leaf to form this shelter around the growing larva, within the middle of the filaments in the picture.
No galls were harmed in this photograph, at least by me. I found this gall already breeched on the ground. I suspect it blew off the tree in the recent stiff winds and was damaged in the process.
The camera was a Nikon D90; the lens Micro-Nikkor 60mm AF. Post processed in Aperture.