Sexual galls are in oak twigs, and are hard and woody, causing knobbly swellings, which can persist for some time after being vacated. Inside is a long narrow larval chamber. 

Similar galls in twigs are formed by the agamic Andricus curvator.  This has a small spherical inner gall

We recommend that the gall is verified by an expert before submitting to NatureSpot - take good photographs, note the host species and, if possible, retain a specimen. To distinguish from other twig galls, it may be necessary to open the gall. 

native oaks

Common (British Plant Galls, Redfern & Shirley, 2023)

occasional

see British Plant Galls (Redfern & Shirley, 2023)