The inconspicuous sexual galls are found in Turkey oak catkins, and are small (1.5mm - 2mm) and rounded/cone-shaped; green at first and then brown.  

Andricus grossulariae sexual galls also form on Turkey oak catkins, but are much larger and brightly coloured

We recommend that the gall is verified by an expert before submitting to NatureSpot - take good photographs, note the host species and, if possible, retain a specimen

Turkey oak.  It is found on other species of oak, but not yet in the UK 

May

common (British Plant Galls, Redfern & Shirley, 2023)

probably common, but overlooked and under-recorded

See British Plant Galls (Redfern & Shirley, 2023)