Osmia (Osmia) emarginata are black bees, sometimes with a metallic blue shimmer to the body (Amiet et al. 2004). Females have intermixed black and pale hairs on the face, yellow-white hair on the thorax, pale hair on T1–T3, both black and pale hair on T4, and primarily black hair on T5–T6. Male hair coloration is similar to the females except for the head, which is primarily white on the face and often has intermixed black and white hairs on the vertex. Female body length is 13–15 mm, and male body length is 10–12 mm (Banaszak and Romasenko 1998).
Osmia emarginata is similar enough to O. mustelina that it could possibly be considered a subspecies of O. mustelina (Peters 1978). Females can be more easily distinguished by the shape of the apical margin of the clypeus and the absence of an impunctate median ridge on the disc of the clypeus (Banaszak and Romasenko 1998). Males are more difficult to distinguish; in general O. emarginata has fuller, reddish-brown hair on the thorax, the gonocoxite is almost right-angled subapically, and the ventral ridge is more apparent (Peters 1978).
Osmia emarginata adults have been recorded in flight from May to July (Banaszak and Romasenko 1998).
Osmia emarginata are known to nest in fissures between stones and cavities in rock, more often in broad cavities than narrow ones (Banaszak and Romasenko 1998). Nest cells are arranged in an irregular, clump-like shape (Banaszak and Romasenko, 1998). Nest cells are composed from chewed leaves (Amiet et al. 2004).
Osmia emarginata specimens have been recorded in North Africa and South and Central Europe (Banaszak and Romasenko 1998).