4 mm long. Dark brown-black, males are darker than females. The upper surface is covered with golden hairs and the femora are dark.  Species level identification of Psallus based on external features is difficult and frequently impossible, especially for females. Noting the host plant may narrow down the range of possibilities.

Other Psallus, especially P. perrisi. 

This species cannot be separated from Psallus perrisi (which also occurs on oak) without dissection of the male genitalia. The male aedaegus has a long, broadly curved sub-apical process (c.f. the relatively short sub-apical process in P. perrisi). 

On Oak. 

Spring-Summer.

Widely scattered (British Heteroptera checklist (2021) - https://www.britishbugs.org.uk/systematic_het.html 

Rarely recorded in Leicestershire and Rutland.