Mason bees are named for their habit of using mud or other "masonry" products in constructing their nests, which are made in naturally occurring gaps such as between cracks in stones or other small dark cavities.

Most occur in temperate habitats within the Palearctic and Nearctic realms, and are active from spring through late summer.

Osmia females typically nest in narrow gaps and naturally occurring tubular cavities.

[1] Commonly, this means in hollow twigs but can be in abandoned nests of wood-boring beetles or carpenter bees, in snail shells, under bark, or in other small protected cavities.

[8][9] Most Osmia species are found in places where the temperature drops below 0 °C (32 °F) for long durations and they are well-adapted to cold winters; chilling seems to be a requirement for maturation.

[2] Some species of mason bees are semi-voltine, meaning that they have a two-year maturation cycle, with a full year (plus) spent as a larva.

[1] Osmia share a basic anatomy with all bees and most insects; the main functional regions being the head, thorax, and abdomen.

Although the scopa is usually located on the legs in most bees, it lies underneath the abdomen for Osmia and other genera in the family Megachilidae.

Osmia typically pollinate early spring flowers in the family Rosaceae, and will even forage under poor weather conditions.

[13] Commercial pollinators include O. lignaria, O. bicornis, O. cornuta, O. cornifrons, O. ribifloris, and O. californica.