The fungus Taphrina caerulescens  causes galls to form on Red Oak and other oaks. Raised yellow blisters on the upper surface, 1cm+ diameter, becoming brown later; depression underneath lined with asci.  

host

Photographs of upper and lower surface; note the species of oak

Primarily Red Oaks

Frequent in south England on Quercus rubra and related species (ref.: Redfern & Shirley, 2023, British Plant Galls) 

Rare