The asexual gall produced by this wasp in the buds of native Quercus species is said to resemble the horns of a Ram in its shape.  The gall commonly forms a 'chimera' with another gall-causing species, causing oddly-shaped galls, and the presence of the inquiline wasps Synergus spp can also change the shape of the gall, causing enlargement of the bulbous base.

[Note: The sexual gall is reported to be on the catkins of evergreen or Turkey Oak; we are not aware of any descriptions of this gall or of its occurrence in the UK.] 

Photograph the gall on oak, and note the host species

Oak tree

Unknown in England until its appearance in Berkshire in 1997, it is becoming more common and extending its range.

fairly common

See British Plant Galls (Redfern & Shirley, 2023)