Tarspot Fungus
A tarspot fungus on an oak twig: probably Diatrypella quercina.
I initially though this was probably the tarspot fungus Diatrypella quercina but in the comments Ingo Wagner pointed out that it is actually Peniophora quercina.
The spores (x600) are about 9 x 3um:
Specimen taken in Whiteknights Park, Reading, UK, on 2012-12-07.