This morning I stumbled across these photos lying forgotten on my phone since a wonderful late summer walk in the Essex/Suffolk countryside back in August. There were lots of these galls on the oaks we passed, along with lots of healthy looking acorns.
I particularly liked this (slightly blurry!) one, where you can see the tinge of red..
I didn't know anything about oak galls, except that they were used for making ink in medieval times and had a lot of tannin, so I did a bit of reading up and thought other folks might also like to see the links I found. I had no idea that there were so many different types and was glad to discover that the wasps that make them don't pose any real threat to the trees.
Here's what the RHS have to say, here's a picture of a 'Knopper Gall' which seems the closest to my photos (double click to enlarge)
Here are Wikipedia entries on the 'Knopper Gall' in particular and Gall Wasps in general
and here is a link to the British Plant Gall Society with instructions for recording oak gall sitings.