Osmia (Osmia) longicornis are black bees, with metallic blue luster and apical hair bands on the terga (Peters 1978). Females have intermixed black and brown hair on the head and thorax (Peters 1978). T1–T3 have primarily pale brown hair, and T4–T6 often have intermixed black and brown hair (Peters 1978). Males have similar hair to the females except that the margins of T6–T7 have long white hair laterally (Peters 1978). Female body length is 13– 15 mm, and male body length is 11–13 mm (Ducke 1900).
Osmia longicornis are very similar looking to O. opima, especially the females, which previous authors have been unable to differentiate (Romankova 1985). Males can be differentiated by dissecting them to view the gonostylus, which is apically expanded in O. longicornis and not in O. opima (Romankova 1985). S8 also varies and has recurved lateral projections about mid-way through in O. opima and not in O. longicornis (Romankova 1985). S8 of O. longicornis is also acutely pointed apically (Romankova 1985).
Osmia longicornis adults have been recorded in flight from February to April (Peters 1978).
Osmia longicornis hosts are largely unknown, however, pollen loads recorded in a study by Haider et al. (2013) suggest preference for Rosaceae and Salicaceae.
Osmia longicornis specimens have been recorded in Uzbekistan and Afghanistan (Peters 1978).