Chenopodium oahuense subsp. oahuense is a scentless to weakly scented shrub, occasionally taking on a tree-like form, with heights ranging from 50 to 300 cm and sometimes up to 300 cm. The stems are erect but can also be ascending or prostrate and are branched. These stems are usually densely covered with a grayish mealy pubescence, although the younger growth may be greener and only sparsely mealy pubescent. The leaves are simple and alternate, with broadly deltate (triangular) to rhombic blades measuring (1.5–)2–4.2(–10) cm in length and (0.6–)1.3–2.5(–5.5) cm in width. The base of the leaves ranges from truncate to cuneate, and the surfaces are densely mealy pubescent, with the upper surfaces being less so and greener than the lower surfaces. The blades are thick and somewhat fleshy. The leaf margins are usually 3-lobed, sometimes sinuate-dentate, with obtuse to rounded lobes or teeth. The petioles are (0.5–)1.3–2.5(–4.5) cm long, and the plant lacks stipules. The flowers appear in small, dense glomerules grouped into large, terminal, leafless, or nearly leafless panicles. The flowers are bisexual (perfect) and small, with a calyx about 0.8 mm long that completely encloses the fruit at maturity. The calyx lobes are ovate and mealy pubescent, with scarious margins, especially towards the apex. The plant does not have petals. It has 1 to 5 stamens, with dithecal anthers that open by longitudinal slits. The ovary is superior, 2 to 5-carpellate, 1-celled, with one ovule and basal placentation. There are 2 to 5 styles with dry stigmas. The fruit is a simple dry utricle, with each fruit containing one dark brown seed that is approximately 0.8 mm in diameter. The seed surface is papillose, and the margins are indistinct. It typically inhabits dry areas, including coastal regions, dry forests, and subalpine shrublands, at elevations ranging from sea level to 2520 meters.