Clermontia oblongifolia subsp. oblongifolia is a shrub or tree species that grows 2 to 7 meters tall and is found in terrestrial and epiphytic environments. It has light gray or light brown stems, branched repeatedly and spread out, displaying a solid pith with white, viscous latex. Its leaves are simple and alternate, with oblong or elliptic blades measuring 8–20 cm in length and 2.3–5.4 cm in width, having dark green, glossy upper surfaces and very pale green, dull lower surfaces. The inflorescences can be either glabrous or pubescent, with peduncles ranging from 1 to 4.5 cm in length and pedicels from 1.3 to 4.5 cm. These flowers are bisexual, epigynous, zygomorphic (bilaterally symmetric), resupinate, and protandrous, featuring a hypanthium that is 0.8–1.4 cm long. The calyx is synsepalous with five green, firm, valvate lobes that mimic the shape and texture of the corolla. The perianth is 5.5–7 cm long, with a rounded or obtuse apex in the bud, and the lobes are 1.2–2 cm long. The stamens are syngenesious, with white or greenish white anthers that open introrsely. The ovary is inferior, with two carpels and two locules, leading to a slender, terete style and a two-lobed stigma that becomes receptive after pollen release. It produces numerous tiny, dark brown or black seeds, which are minutely foveate-reticulate and contain a straight, short to spatulate dicotyledonous embryo within oily endosperm. It is predominantly found in montane rain forests within the Wai’anae and Ko’olau Mountains, between 395-1200 meters.