This member of the molle complex occurs mostly in Tex., Okl. and Ark., but it also extends locally into Tenn., Ky. and perhaps elsewhere in southeastern states, where it has been much confused with the closely related O. occidentale Mack. (D. Estes, pers. comm.). Both taxa may be relatively distinct from hispidissimum and molle in their larger flowers (11-20 mm long) and more tapered (versus collared) nutlet bases (Turner 1995; Y, W and their citations). O. occidentale is a more northern and western taxon centered in the Great Plains, with relatively large nutlets (4-5 mm), and short pubescence (only 1-2 mm on stems); 2n = 28? (Y)..