Peperomia oahuensis is a species of fleshy herb characterized by its green or reddish-tinged, zigzag stems that ascend from a repent base, typically reaching lengths of 5–30 cm and diameters of 2–3.5 mm at the base. The stems may be unbranched or branched above the base, featuring internodes ranging from 0.4–6 cm long with a small ridge along opposite sides. The leaves are simple and alternate, though some may be opposite. Leaf blades are elliptic-lanceolate to elliptic, sometimes lanceolate or rarely narrowly subrhombic, measuring 4.5–12 cm long and 1.4–3 cm in width. They have long-acuminate to acute apices and cuneate bases. The surfaces are glabrous, with the upper surface medium green and the lower surface pale green. The leaves are relatively thick and firm-fleshy. The margins are flat, and the leaves are palmately 3-veined, sometimes 5-veined, with inconspicuous outermost veins. The petioles are 0.3–1 cm long and glabrous. It produces flowers in spikes, which are 1–3-clustered, usually axillary and sometimes also terminal. The spikes measure 4–7 cm in length with a rachis diameter of 1.5–2 mm, and the flowers are well-spaced. The peduncles are 0.6–1 cm long. Each flower, which lacks a perianth, is subtended by a peltate bract. There are no sepals or petals present, and the flowers contain 2 stamens. The ovary is superior, obovoid, and 1-celled with an oblique apex. The fruits are broadly ovoid, measuring 0.6–0.8 mm long, and are papillose-viscid with sessile glands. Each fruit contains one seed with scanty endosperm and copious starchy perisperm. It is found in mesic valleys between 300–550 meters.