Asterodiaspis sp.

Asterodiaspis species are scale insects.  3 species cause a 'pit-gall' on oak twigs: Asterodiaspis quercicola, A minus and A variolosa.  They can't be identified from the galls.  To identify the nymphs and adult females, very detailed microscopic examination is needed; see https://core.ac.uk/download/pdf/76993921.pdf

The scale insect creates a small pit in the twig surrounded by a raised rim, c. 2mm across.  The scale insects are greenish or brownish, and sit in the middle of the pit.  

Gall insect

These can be recorded on oak; your photos must show the oak twig with buds/leaves as well as the galls

common, according to British Plant Galls (Redfern & Shirley, 2023)

not known - few records