Andricus quercustozae

Andricus quercustozae (Bosc, 1792) is an oak gall wasp species widely distributed in the Mediterranean region. Brownish hard-shelled and unilocular asexual generation galls are formed by A. quercustozae on buds of oak trees in the late summer and early fall. It is one of the biggest oak galls, reaching a diametre of 35 mm.
This site is in French, but there are some photos of the different stages of the development of the gualls.
http://www.insectes-net.fr/galles/galle8.htm