The common spangle gall houses the larvae of a gall wasp (Neuroterus quercusbaccarum); each gall containing one wasp larva. It is a species linked solely to oak tress (hence the quercus in the scientific name) and occurs where the wasp has laid an egg on the underside of an oak leaf. There can be lots of galls on one leaf but not all leaves will be affected of course. Each gall is a fairly flat packet about 5mm across with a central raised pimple. The centre of the gall can appear reddish in colour thanks to the presence of many red hairs, the disc of the rest of the gall tends to be yellowish or pale green.
When the leaf falls from the tree in autumn the larvae feed in the leaf litter over winter and the adult wasps emerge in the spring to start the next cycle of the insect's development. Galls are really quite complicated and often have sexual and asexual generations. This gall is the asexual stage of the cycle where the larvae that hatch in spring are all pregnant females! Now that is something to get your head around!