Apparently there are about 30 species of Gall wasps. These particular mutations are known as knopper galls and are made by the gall wasp Andrinus quercuscalicis.

There is a link below if anyone is interested in knowing more. It's a fascinating read, if you like this sort of thing.

Personally I think that these look uncannily like skulls! Try viewing large for detail.

staging.sussexwildlifetrust.org.uk/news/oak-galls