Osmia tricornis are black bees with a metallic dark blue luster to the head and thorax and a coppery luster to the abdomen (Peters 1978). Females have dark brown hairs on the face, black hair on the thorax which can be reddish on the scutellum, and the metasoma hair is an intense rusty red (Peters 1978). Males have yellowish to rusty red hairs which are lighter on the head and darker on the metasoma (Peters 1978). Female length is from 13–15 mm, and male body length is 9–10 mm (Ducke 1900).
Osmia tricornis adults have been recorded in flight from mid-May through to mid-July (Vicens et al 1994). O. tricornis flight time coincides with the decline of nesting activity of O. cornuta (Vicens et al 1994).
Osmia tricornis are known to harvest pollen from plants that produce little to no nectar. O. tricornis have preference for three different plant families: Cistaceae, Papaveraceae, and Fabaceae (Vicens et al 1994).
Osmia tricornis are known to nest in preexisting cavities. They have been observed nesting in empty snail shells, drilled borings in wooden blocks, hollow stems, and insect burrows in the ground or dead wood, including abandoned cells in exposed nest of Chalicodoma (Vicens et al. 1994).
Osmia tricornis specimens have been recorded in the foothills of the Italian Alps, as far north as Lyon in France, and across Tunisia, Morocco, and Libya (Peters 1978).