A galling sight

What are these curious things on the oak leaves? Not a disease, but small constructions made by the oak tree around where a wasp larva has been deposited. When the leaves eventually drop off they overwinter in the leaf litter to emerge in the spring, or so the internet tells me! There are a huge variety of these galls out there to go with the huge variety of wasps, most of which humans don't notice. with over 7,000 species apparently.