The sexual generation gall is in oak leaves or occasionally in a twig; rarely in catkins.  In the leaf, it is roughly globular, about 8 mm across and often distorts the leaf and midrib; sometimes several galls coalesce. In a twig, it causes a swelling and usually a bend in the twig, with a spherical inner gall.  

The sexual galls of Andricus inflator are also in a twig, but with a long and narrow interior cavity; to record these two galls the gall needs to be opened.

Photograph the gall on oak, and note the host species

oak trees

June and July

common

See British Plant Galls (Redfern & Shirley, 2023)