Wikstroemia oahuensis var. oahuensis ranges from a low shrub at lower elevations to a slender or robust tree that can reach heights of 4(–6) meters. The main stem may be short, slender to robust branches, and ± erect. The stems can be glabrous, pubescent, or glabrate and are sometimes marked with whitish lenticels near prominent leaf scars. The leaves are simple and opposite, with blades ranging from narrowly to broadly elliptic or ovate-elliptic and occasionally slightly obovate. They measure (1–)2.5–8(–20) cm in length and (0.5–)1.5–3(–6.5) cm in width. The leaf apex varies from acute to rounded, sometimes acuminate or slightly apiculate, while the base ranges from acute to rounded, occasionally subcordate. The lower surfaces often feature conspicuous lateral veins and can be glabrous or pilose, especially along the midrib. The upper surfaces are pale to dark green, occasionally tinged with lilac or crimson, and sometimes appear glaucous. The margins are entire, and the petioles range from (2–)4–9(–13) mm in length and can be thin or stout, glabrous or pilose. Stipules are absent. The flowers appear on peduncles 2–15(–25) mm long, which can be glabrous or pilose, sometimes recurved. The rachis is subglobose, conical, or filiform, rarely forked, measuring 0.4–5 mm in length, and can extend up to 12 mm and then pendulous, glabrous, or puberulent. The flowers are either bisexual or unisexual and sometimes appear bisexual, green to yellow, and can be glabrous or hirsute. The calyx tube varies significantly in length and lobe dimensions between staminate, perfect, and pistillate flowers. The corolla is absent, and stamens are usually in two whorls, with the lower included and the upper sometimes shortly exserted. The ovary is superior and 1-celled with a solitary, pendulous ovule. The fruit is orange to red and ellipsoid, measuring 6–18 mm long and 4–10 mm in diameter. The seeds lack endosperm. It is found in hala (Pandanus tectorius) forests, mesic to wet forests, bogs, and on ridges and rocky ledges between (5–)100-1,400 meters.