Knopper Galls

This oak tree along the Ladygrove meadow has once again inflicted by gall wasps.

Being a turkish oak, a gall wasp will inject it's eggs into a forming acorn and the associated chemicals turn the acorn into a gall (in this case a knopper gall) - a source of food and protection for the growing larvae
Possibly, the small insects on the first picture are the emerging gall wasps:

More information on galls can be found at the UK Safari website