The agamic (asexual) galls are large artichoke galls formed in oak buds -  much enlarged (up to 30mm long) and covered with overlapping bud scales.  There is a cavity inside containing the the inner gall which is ejected in late summer, leaving the old galls to persist with the bud scales opened out and brown. 

[Note: The sexual generation of galls are in the catkins and are small and covered in long white hairs.]

Photograph the gall on oak, and note the host species

native oak trees

common

See British Plant Galls (Redfern & Shirley, 2023)