What are these balls my oak tree is dropping?

[GrannyAchingsShepherdsHut]

My very old (100s of years) oak tree is dropping catkins as expected, but attached to the catkins are loads of these green, juicy, slightly transparent balls.

What on earth are they? Do I have particularly stupid gall wasps that have laid on the catkins for some reason?!

Never seen anything like it before but they're everywhere, 100s of them.

Looks like a gall of some kind. Try currant gall

Thank you! That is indeed what they are!

I was googling away with no luck at all.

I've now learnt more than I wanted to know about the sex life of a gall wasp. So that's nice.