Lobelia oahuensis is an erect, unbranched shrub that reaches heights of 1 to 3 meters. It has woody stems that are 1 to 3 m long and end in a dense apical rosette of leaves. The leaves are simple, alternate, and have elliptic blades measuring 40 to 60 cm in length and 4 to 6 cm in width. The leaf apex is acuminate, and the upper surfaces are glabrous, while the lower surfaces are densely covered in grayish or greenish hirsute hairs. The leaf margins are callose-denticulate and revolute, and the leaves are sessile or subsessile without stipules. The flowers are arranged in inflorescences 3 to 5-branched from the base, extending 100 to 150 cm. These inflorescences are pubescent, with pedicels measuring 10 to 20 cm long. The linear bracts are 20 to 40 mm long and 2 to 3 mm wide, with acuminate tips. The hypanthium is obconical to obovoid, 4 to 6 mm long, and covered in pubescence. Each flower is bisexual (perfect), featuring a calyx with five linear, acute lobes 12 to 18 mm long and 2 to 3 mm wide. The sepals are connate, with the tube adnate to the ovary. The corolla is zygomorphic (bilaterally symmetric) and dorsally cleft nearly to the base, pale blue, 42 to 45 mm long, 4 to 5 mm wide, and pubescent or glabrous. The corolla lobes are spirally revolute, and the tube is suberect. There are five stamens alternating with the corolla lobes. The staminal column is pubescent, and the anthers are pubescent or glabrous, with the lower two featuring apical tufts of white hairs. The anthers are dithecal and open via longitudinal slits. They are coherent but can separate after anthesis or remain connate, forming a tube into which pollen is shed. The filaments may be distinct or connate above and are attached to the epigynous nectary disk or the base of the corolla and, in rare cases, adnate to the corolla tube. The ovary is inferior, 2-celled, often topped with an annular nectary disk, and the placentation is axile or, when 1-celled, features two parietal placentas. The stigmas are 2-lobed, initially appressed and nonreceptive, as the style grows through the anther tube to push out the pollen, after which they spread apart and become receptive. The fruit is an obovoid capsule, 10 to 17 mm long and 8 to 10 mm wide, pubescent, and opens at the apex by two valves. The seeds are numerous, smooth, brownish, 0.3-0.5 mm long, with a straight, short to spatulate dicotyledonous embryo embedded in oily endosperm. It is found in cloudswept wet forests on the windswept summit of the Ko’olau Mountains on O’ahu, typically at elevations ranging from 850-920 meters.