Description from Flora of China
Shrubs or small trees. Stems fleshy, usually many branched, terete, club-shaped, subglobose, laterally compressed; areoles with glochids and usually 1 to many spines. Leaves conic to terete, usually small, caducous. Flowers solitary, lateral or subterminal, rarely terminal, sessile. Receptacle obovoid, truncate and depressed at apex. Perianth rotate, spreading, or erect, inserted at rim of receptacle tube; segments numerous, outer ones sepaloid, inner ones petaloid. Stamens inserted in perianth throat, sensitive (except in O. cochenillifera). Ovary (pericarpel) inferior; placentas parietal. Fruit fleshy or dry, globose or ovoid, umbilicate, with areoles, glochids, and sometimes spines. Seeds encased in a white, hard, rarely hairy aril.
About 90 species: Americas; at least 30 species cultivated in China, of which four naturalized in southern and southwestern areas.