Osmia melanocephala are black bees with a slight metallic blue hue on the head and thorax and a metallic greenish hue on the abdomen. Females have black to dark brown hair on the head and thorax and yellowish-red hair on the abdomen (Wu 2006). Males are densely covered with red-yellow hair on their thorax and have light reddish-brown hair on their abdomen (Peters 1978; Wu 2006). Female body length is 10–13 mm, and male body length is 10–11 mm (Peters 1978; Wu 2006).
Osmia melanocephala females can look similar to O. cornuta and O. kohlii that have entirely dark brown or black hair on the head and thorax and reddish hair on the abdomen. O. melanocephala are easily distinguished by their simple, unmodified clypeus (Peters 1978). Males can look similar to species like O. taurus that can sometimes have entirely pale red hair on their abdomen, but O. melanocephala can be differentiated by the long first flagellar segment (Wu 2006).
Osmia melanocephala adults have been recorded in flight from the end of May to the end of June (Peters 1978).
Osmia melanocephala are polylectic and have been observed collecting from 5 different plant families, with a strong preference for Fabaceae. Other known pollen sources are Rosaceae, Lonicera, and Boraginaceae (Haider et al. 2013).
Osmia melanocephala specimens have been recorded in Xinjiang, China, northwestern Mongolia, and Turkmenistan (Wu 2006).