Osmia (Osmia) mustelina are black bees with a metallic blue shimmer (Amiet et al. 2004). Females have pale hair on their face, sometimes with abundant black hair intermixed on the frons and vertex (Peters 1978; Fig. 1). Their thorax has mostly pale hair (Peters 1978; Fig 2). T1–T3 can be covered dark brown to white hair, T4 can have white to black hair, and T5–T6 usually have entirely brown or black hair (Peters 1978; Fig 3). Male hair is similar to females except that generally males are hairier than females (Peters 1978). Female body length is 12–16 mm, and male body length is 10–12 mm (Amiet et al. 2004). O. mustelina found in Turkey tend to be much brighter than their counterparts (Peters 1978).
Osmia mustelina is similar enough to O. emarginata that O. emarginata could possibly be considered a subspecies of O. mustelina (Peters 1978). Female O. mustelina can be easily distinguished by the shape of the apical margin of the clypeus and the presence of an impunctate median ridge on the disc of the clypeus (Banaszak and Romasenko 1998). Males are more difficult to distinguish, in general O. mustelina has less full and paler hair on the thorax, the gonocoxite is obtusely angled subapically, and the ventral ridge less apparent (Peters 1978).
O. mustelina adults have been recorded in flight between April and July (Peters 1978).
Osmia mustelina is known to collect pollen from Fabaceae, Cistaceae, Boraginaceae, Plantaginaceae, Rosaceae, and Ranunculaceae (Haider et al. 2013).
Osmia mustelina is native to Europe; they occur in the warmer areas of Central Europe. Distribution occurs in Italy, Germany, Switzerland, Hungary, Crimea, Greece, and Turkey (Peters 1978).