The agamic galls form on terminal and occasinally lateral buds of native oaks.  They are large spherical galls that start green and become brown and very hard, often with small surface bumps.  They can persist for several years.  There is a single causer larval chamber inside, although inquilines are also common in the gall tissue. 

[Note: The small inconspicuous sexual gall is found in the buds of the Turkey Oak.  It is ovoid with a pointed tip, but is difficult to separate from the sexual galls of Andricus lignicola and A corruptrix].

Andricus infectorius galls on oak. 

Photograph the gall on oak, and note the host species

native oaks

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

Common

See British Plant Galls (Redfern & Shirley, 2023)