The oak processionary (OPM) is a moth whose caterpillars can be found in oak forests, where they feed on oak leaves, causing significant damage. They travel in nose-to-tail processions, often arrow-headed, with a leader followed by rows of several caterpillars abreast. They are a human irritant because of their venomous setae, which can cause skin irritation and asthma. The species was first described by Carl Linnaeus in his 1758 10th edition of Systema Naturae.

An oak is a hardwood tree or shrub in the genus Quercus of the beech family. They have spirally arranged leaves, often with lobed edges, and a nut called an acorn, borne within a cup. The genus is widely distributed in the Northern Hemisphere; it includes some 500 species, both deciduous and evergreen. Fossil oaks date back to the Middle Eocene. Molecular phylogeny shows that the genus is divided into Old World and New World clades, but many oak species hybridise freely, making the genus's history difficult to resolve.