Melicope oahuensis is a shrub to shrubby tree ranging from 2 to 7 meters in height. The new growth on these plants is yellowish-brown, puberulent with a waxy scurf, but it becomes glabrate fairly quickly. The stems are usually highly branched. The leaves are simple and opposite. The blades are elliptic–obovate, elliptic, or broadly oblong-elliptic, measuring (4.5–)6–17 cm long (often less than 12 cm long) and 2.5–8 cm in width. The apex is rounded and often slightly emarginate, while the base ranges from cuneate to subtruncate. Leaf surfaces are mostly glabrate, except for the midrib on the lower surfaces, which may retain a few scattered hairs. When crushed, the leaves emit a strong, pungent anise odor and are thin yet coriaceous. Margins are entire and can be flat to sometimes strongly revolute. The primary lateral veins, 7–15 pairs, are ascending and connected by a weakly arched vein 1–5(–9) mm from the margin. Higher-order venation forms a prominent reticulate pattern. Petioles are 6–45 mm long, and stipules are absent. The flowers are generally 3–5(7) per plant, arranged in short, glabrate to sparsely puberulent axillary cymes typically less than 2 cm long. Peduncles are about 3–4 mm long, and pedicels are approximately 3 mm long. Bracteoles are minute, deltate, and may be puberulent or glabrate. Flowers are functionally unisexual or bisexual (perfect), with staminate and perfect flowers often found on the same plant. The calyx consists of 4 sepals, each deltate–ovate and 1.8–2.2 mm wide, minutely ciliate but otherwise glabrous. Petals are valvate and deltate, measuring 4.5–6 mm long and glabrous. Eight stamens are arranged in two whorls, and they are reduced but always present in pistillate flower functionally. The ovary is superior and approximately 1 mm long, with a slender, glabrous style about 2 mm long. The ovary is 4-celled and lobed, with connate carpels but slightly distinct at the apex. Each cell contains two hemitropous ovules. The fruit is a depressed cuboid to depressed cuboid globose capsule about 9 mm long and 10–14 mm wide, and it is either unlobed or weakly lobed. The seeds are 1(2) per carpel, about 6–7 mm long, glossy black when ripe, crustaceous, and ovoid, sometimes angled from compression. The embryo is straight, embedded in a fleshy endosperm, with a short radicle and compressed, ovate cotyledons. It is found in mesic to sometimes wet forests between 430 – 880(–1,070) meters.