O. callistachyum is a perennial herb or shrub to 3.5 m (5 m) tall; young stems quadrate to quadrasulcate, glabrous or sparsely pubescent. Leaves elliptic to elliptic ovate, 15-30 cm long and 4-12 cm wide, apically acuminate with the tip often curved to one side, basally attenuate, glabrous above, puberulous beneath along the midrib and the major costa, the margins entire to crenulate; petioles wanting or to 1 cm long. Inflorescences in terminal racemes 30-60 cm long, the lateral peduncles to approximately 5 mm long, the flowers in fascicles, each fascicle sub- tended by a subulate, keeled, puberulous bract 2-3 mm long, the flowers mostly borne in dichasial cymes, each subtended by a similar, though slightly smaller, more triangular bract; rachises subtomentose to tomentose; pedicels 3-6 mm long, puberulous to tomentose. Flowers with the calyx 5-merous, the segments equal, subulate, keeled, puberulous, 2-3 mm long; corolla pinkish purple, bilabiate, funnelform, often curved to one side, to 3.0 cm long, 5 mm wide at the throat and 2 mm wide at the base, the tube puberulous on both surfaces, the upper lip 6-7 mm long, 4.5 mm wide at the base, 2-lobed, the lobes ovate, 4.5 mm long, 2.5 mm wide, apically obtuse, the lower lip of 3 lobes, each 7-8 mm long, 2 mm wide, apically obtuse; stamens extending to or just beyond the notch of the upper lip, the filaments villous, the staminodes to 3.5 mm long, the apex slightly enlarged, curved and apiculate, the sterile anthers puberulous, the filaments glabrous. Capsule 1.5-2.5 cm long, glabrous. Seeds subcordate in outline, 3-5 mm long, 2.6-3.3 mm wide, surface rugose
