Clustered slender to moderate palm climbing to 15-30 m. Stems without sheaths 8-16 mm in diameter, with 12-28 mm; internodes 12-18 cm long. Leaf sheath longitudinally striate, dark green, tan, often dull crimson brown on young sheaths, moderately to sparsely armed with brown-black spines, concentrated and persistent on the ocrea, spines often sloughing off elsewhere on sheath to leave conspicuous, raised, circular, blister-like scars; thin, white caducous indumentum present on mature sheaths, particularly dense on young sheaths and juvenile individuals; ocrea ± truncate or saddle-shaped with a 0.5-1.8 cm high rounded lobe opposite the rachis, armed as the leaf sheath, spines concentrated at ocrea margin, extending ± 2 cm. Spear leaf deep orange to bright crimson. Juvenile stems up to 6 m long, with sheaths, less than 1 cm in diameter, leaves sessile or with a short (1.5 cm) petiole, strongly bifid, eventually becoming pinnate, up to 30 cm long and 4.5-6 cm broad at the widest point and with a 60 cm long cirrus emerging from the centre; elaminate rachis often present at base of stem, up to 2.5 m long. Leaves on mature stems sessile, or with a short (less than 2.5 cm) flattened petiole; rachis 1-1.2 m long, abaxially rounded, adaxially concave, becoming circular in cross section distally, armed as the leaf sheath, spines particularly profusely armed on underside, although often smaller than those on the sheath; cirrus 1-1.5 m long, unarmed; leaflets, 25-35 on each side of the rachis, linear-lanceolate, to mildly sigmoid, broadly attenuate at the base, apex somewhat acuminate, 16-32 cm long, 1.8-2.5 cm broad at the widest point, ± concolourous, dark green, armed along the margins with robust spines particularly at base of leaflet, single-nerved, with 6-7 conspicuous secondary nerves on each side; lowermost leaflets smaller than the rest, held vertical to rachis; acanthophylls up to 1.5 cm long. Inflorescences in successive axils 3-5 m from stem apex; peduncle 8-15 cm long, hemispherical in cross section; prophyll 3-8 cm long; peduncular bracts 2-4, 2.5-4 cm long; rachis up to 0.8-1 m long, pendulous; rachis bracts 2.5-4 cm long; rachillae circular in cross section, 15-28 cm long, pendulous, bracts deep-bright crimson prior to anthesis, becoming dry, brown; prophyll subtending flower cluster 3-5 mm, conspicuously striate. Flower cluster with 1-3 central pistallate flowers subtended by 2 lateral cincinni of 1-2 pistillate and 2-3 staminate flowers; staminate flowers 5.5-6.5 mm by 2–3 mm; calyx ca. 5.0 mm by 5.0-5.5 mm, including 1 mm long stalk, tubular for half to 3-quarters of its length; corolla ca .5 mm long, tubular for the basal ± 1 mm, cream/yellow; staminodial tube ca. 2 mm long; anthers 0.8 mm by 0.3 mm; pistillode 1.5 mm by 0.7 mm, thin, tapering; pistillate flowers similar to the staminate flowers, up to 3-4 mm wide; ovary ca. 2.5 mm by ca. 1.5 mm. tipped by 1-1.5 mm long style. Fruitat maturity, globose to sub-globose, 1.8-2.2 cm by 1.6-2 cm, with 15-17 rows of vertical scales. Seed sub-globose 1.2-1.6 cm by 1.5-1.9 cm, with a narrow rounded depression below, covered with regular polygonal depressions, giving a distinctly warty appearance; sarcotesta white, thin (less than 0.5 mm).
Oncocalamus mannii is common in open areas, roadside and forest gaps. This species responds extremely well to selective logging and is a common component of regrowth vegetation.
Native to, Cameroon, Congo, Equatorial Guinea, Gabon, Nigeria, Zaire
Oncocalamus mannii is restricted from southern Cameroon to Gabon.