These are oak galls. They’re quite common and are found on many oak species and come in various different shapes. I’ve found dried, brown ones before but have never seen red ones like these before. You can see they’re stuck to the underside of a leaf so aren’t very big – about 2cm diameter. 

They are made by the oak itself and form as a reaction to an adult female gall wasp injecting chemicals into the tree. Different gall wasps produce different galls. The wasp lays an egg inside the gall and the larvae eats the fleshy insides. Eventually it changes into an adult wasp and leaves.