Length 3.7 - 4.6; smaller than other two species on Birch; females larger than males.  Sexually dimorphic and variable in colouration and intensity of markings. There is usually a band across the face between the eyes, darker in males.  Males have grey/brown pronotum, chestnut brown scutellum often with 2 black basal triangles; smoky forewings with black-brown veins, a more or less rounded brown spot on the cross-vein (m-cu) of the forewing, and darkened usually apical cells.  Females have chestnut brown vertex, pronotum and scutellum, and brown wings often with hyaline patches; they have a short ovipositor.

other Oncopsis

Detailed examination of the specimen is needed.  Photograph specimen in side view, top down and face; note length and whether male/female.  You must note the host plant from which you collected the specimen.

Birch

Adult: June to September.

Common. (ref.:  UK Auchenorrhyncha checklist 2021, on British Bugs website)

Rarely recorded; likely to be under-recorded due to ID difficulty.

https://www.britishbugs.org.uk/homoptera/Cicadellidae/Oncopsis_tristis.html