| 1 | Secondary veins arcuate ascending, not more than 20 pairs, not joining, not forming a submarginal vein |
| 1' | Secondary veins straight to rather arcuate ascending, 10 to more than 40 pairs, joining, forming a submarginal vein |
| 2 | Leaves in whorls of 2-4; blades papyraceous. Inflorescence 4-9 cm long. Sepals ovate; stamens inserted at 0.7-0.8 of the length of the corolla tube |
| 2' | Leaves in whorls of 4-6(-7); blades coriaceous. Inflorescence 9.5-18.5 cm long; sepals suborbicular to orbicular; stamens inserted at 0.2 of the length of the corolla tube |
| 3 | Leaf blades width 1-1.7 cm, ratio 4.2-7.1. Fruits syncarpous |
| 3' | Leaf blades width 2.1-18 cm, ratio 1-5.8. Fruits apocarpous or hemisyncarpous |
| 4 | Leaf blades papyraceous when dried. Peduncle winged |
| 4' | Leaf blades coriaceous or papyraceous when dried. Peduncle not winged (unknown in O. tenimberensis) |
| 5 | Leaf apex acute or acuminate; peduncle 3.5-6 cm long; corolla tube 3.9-4 mm long; mericarps less than 1.5 cm long |
| 5' | Leaf apex cuspidate; peduncle 0.6-3.3 cm long; corolla tube 13 mm long; mericarps up to 4.6 cm long |
| 6 | Calyx ciliolate at margin; fruits hemisyncarpous |
| 6' | Calyx not ciliolate at margin (unknown in O. tenimberensis and O. sciadophylla); fruits apocarpous |
| 7 | Mericarps smooth when dried; endocarps massive, hard, surrounding 2 lateral cavities |
| 7' | Mericarps verrucose to remotely so, or with fine irregular ridges when dried; endocarps loosely fibrous; lateral cavities absent |
| 8 | Secondary veins frequently anastomosing around the submarginal vein; submarginal vein angled, 0.3-1 cm inside from the margin; fibres of the endocarps hard and thick; seeds 3 at either placenta |
| 8' | Secondary veins infrequently anastomosing around the submarginal vein; submarginal vein not angled, close to the margin; fibres of the endocarps rather slender and thin (except for O. oppositifolia, which are slender and thin or hard and thick); seeds 1 or 2 at either placenta |
| 9 | Secondary veins less than 17 pairs. Mericarps length 2.8-4.7 cm |
| 9' | Secondary veins 20 to more than 40 pairs. Mericarps length 4.1-8.5 cm |
| 10 | Leaf apex acute to abruptly acuminate, or sometimes rounded; peduncle 1-3 cm long; corolla tube 6 mm long; sepals ovate, apex rounded, thick; mericarps apiculate at apex |
| 10' | Leaf apex gradually acuminate to cuspidate; peduncle 0.2-1 cm long; corolla tube 7.5-8 mm long; sepals orbicular, apex blunt, thin; mericarps acuminate at apex |
| 11 | Secondary veins arcuate ascending, 20-22 pairs; flowers 10-12 in each inflorescence; seeds 1 at either placenta |
| 11' | Secondary veins straight to rather arcuate ascending near the margin, 21 to more than 40 pairs; flowers more than 30 in each inflorescence; seeds 1 or 2 at either placenta |
| 12 | Leaf blades ratio 2.1-5.8; secondary veins 0.1-0.7 cm spaced; corolla tube 3 mm long; stamens inserted at 1 mm below the mouth; seeds always 2 at either placenta |
| 12' | Leaf blades ratio 1-3; secondary veins 0.2-1.5 cm spaced; corolla tube 5.3-7 mm long; stamens inserted at 2-2.5 mm below the mouth; seeds often 1 at either placenta (or rarely with a small second one) |
