The genera name from Latin operculum; 'little lid' and Greek karyum; 'nut', referring to the lidded, nut-like seeds. The species is named after Raymond Decary, 20th century plant collector. O. decaryi is a thick stemmed tree with more or less straight pubescent branches and usually red flowers. O. pachypus Eggli 1995 is a dwarf pachycaul (one meter high, 50 centimetres in diameter), with strongly zig-zagged smooth branches and yellowish-green flowers. A. Randrianasolo & P. P. Lowry, 2006: "Several additional features separate O. decaryi from other members of the genus whose leaves have a winged rachis, including leaflets that are totally glabrous below, and branches that are straight (rather than zig-zag in orientation, as in O. pachypus)". The one is O. decaryi from Copenhagen Botanical Garden. And it's flowers. These are the flowers of O. pachypus, found at Mbuyu.

