Wingspan 6 to 8 mm. Some more northern examples have rather more blackish suffusion in the wing. Best identified from the similar P. sorbi and P. mespilella by recording as a leafmine and noting the larval foodplant - usually Hawthorn (and occasionally Pear or Quince).. 

Adult Leafmine

Various habitats particularly those where Hawthorn is plentiful.

In common with other species, there are two generations, producing adults in May and in August.

The larval mine is a blotch on the underside of a leaf, often causing the edges of a lobe to curl under.

Widespread and fairly frequent in most areas of Britain. In the Butterfly Conservation's Microlepidoptera Report 2011 this species was classified as common.

Occasional in Leicestershire and Rutland. L&R Moth Group status = C (very scarce resident or rare migrant).

http://www.leafmines.co.uk/html/Lepidoptera/P.oxyacanthae.htm

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/lepidoptera/ditrysia/gracillarioidea/gracillariidae/lithocolletinae/phyllonorycter/phyllonorycter-oxyacanthae/