Material seen. — SABAH. Interior Zone: 1 km SE. of Simatuoh, 10 km ESE. of Sepulot (leg. Dorman, UF I96560/>I0, incl. HOLOTYPE; do. 196682/>10; do. 196685/>10; do. I96691/>10; do. 196697/>10); 5.5 km NNE. of Simatuoh, 9 km E. of Sepulot (leg. Dorman, UF 196551/>10); 2.75 km NNW. of Simatuoh, 7 km E. of Sepulot (leg Dorman, UF 196612/>10).
Spire conical with flat to concave sides. Apex not oblique. Whorls 5 3/8-7 1/8, convex; last whorl rounded or slightly angular. Constriction with or without a longitudinal palatalis, with a transverse palatalis. Tuba gradually narrowed towards the constriction, rounded to slightly angular below. Radial ribs with a shallowly concave to deeply trough-shaped projection half-way, abrading to a scar with a shallow to deep, semicircular loop (projection of those close to the tuba double-channeled with the upper channel much deeper than the lower, abrading to a distinctly sinuous scar or one with a single deep loop when the shell is observed in front view); those on the spire moderately to widely spaced (2-5 ribs/0.5 mm on the penultimate whorl); those on the tuba widely spaced (1-2 ribs/0.5 mm half-way the tuba), below with a slightly curved projection. Spiral striation absent or inconspicuous. Umbilicus open, 0.30-0.40 mm across. Aperture tilted up to 30° with regard to the coiling axis, circular to elliptic. Peristome simple or double; when simple spreading, often with a somewhat thickened lip inside; when double outer peristome somewhat spreading beyond the inner or not, if so gradually narrowed towards, and absent along the right side of the aperture, often somewhat widened along the upper side; inner peristome usually not or hardly protruding from the outer, spreading. Spire: height 1.9-2.6 mm; width 1,3-1.6 mm; index 1.3-1.7. Total width 2.5-2.9 mm. Height aperture 0.6-0.7 mm; width 0.6-0.8 mm.
Distribution. — Borneo: Sabah, Interior Zone, a small area E. of Sepulot.

1. Variable in the shallowly/deeply looped radial ribs, and in the presence/ absence of both a longitudinal palatalis in the constriction as well as spiral striation. These characters occur in all possible combinations in the abundant material available. It is therefore not possible to divide this taxon any further.
2. Three samples from S. of Simatuoh (UF 196682, 196685, 196697) differ in having a simple peristome. In the last mentioned sample such specimens occur together with specimens with a double peristome. A similar variability occurs in 0. obliquedentatum, from the same area.
3. O. anisopterum has a narrower umbilicus; 0. obliquedentatum and 0. inornatum have hardly sinuous radial ribs close to the tuba; 0. hosei and 0. cookei have a spire with a distinctly angular last whorl; 0. cyrtopleuron does not have projections on the radial ribs on the spire.