The cynipid gall wasp causes galls to form on Oak.  The agamic generation galls are usually found low on the main trunk of a smallish tree or sapling, or else on branches near the ground. They are conical and ridged, initially purplish and fleshy, later brown and hard.  

The sexual generation galls affect the buds, but have not been found in Britain. 

Andricus quercuscorticis forms agamic galls on the truck of oaks; they are whitish and ovoid, and the empty sockets persist

Agamic gall Adult

oak trees (usually saplings)

The galls persist for some time after the emergence of the wasps.

Locally common (Redfern & Shirley, 2023)

rare/rarely recorded

Taxonomy is confusing. The species was formerly known as Andricus testaceipes, but this is now considered to be a different species known from the sexual galls found in leaf midribs/petioles; the asexual form of A testaceipes may be the same as the rare A rhyzomae.   See British Plant Galls, Redfern & Shirley 2023 for more information