The agamic form is a medium-sized gall on oak buds. Rounded and may be nearly spherical, up to 15mm diameter, with a narrow attachment to a lateral or terminal bud on oak. Green then becoming reddish or orangey brown, often with light brown warty lumps, which sometimes create a 'crown'. 
The sexual generation forms small galls in Turkey Oak catkins, but has not yet been found in the UK. 

Andricus kollari is usually larger, but sizes are variable.  The narrowing of the base to a narrow point of attachment appears to be a constant feature (ref. Redfern & Shirley, 2023)

agamic gall adult

A photograph of the gall in situ on the oak twig. Note size of gall. It may be necessary to have this record verified by an expert in galls.

Oak trees

Autumn

Scattered records; first recorded from the UK in 2013.  Widespread in the Mediterranean region, and may have been introduced in 18th/19th centuries to provide a source of tannin used in the dye industry and in ink production, and been unnoticed in the UK until recently. (ref. Leach, 2020)

Rare in VC55; first found in 2018.  All records to autumn 2023 are from Charnwood Forest. 

Redfern, M & Shirley, P. 2023 British Plant Galls (third edition). FSC
Leach, C. 2020.  Andricus infectorius (Hartig,1843): Biology, History & its occurrence in Britain. Cecidology 35 (1): 22-26.)

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/hymenoptera/apocrita/cynipoidea/cynipidae/cynipinae/cynipini/andricus/andricus-infectorius/