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Genus Moho
Oahu Oo - The males reached a length of 30.5 centimetres. The wing length was 10.5 to 11.4 centimetres, the culmen was between 3.5 and 3.8 centimetres and the tarsus was between 3.4 and 3.8 centimetres. The females were smaller. The plumage was predominantly sooty black. The tail feathers were brown and had, with the exception of the two central tail feathers, white tips. Further characteristics were the white feather tufts under the axillaries and the two narrow central tail feathers which changed into fine hair-like or fibrous tips. The flanks and the undertail coverts were coloured deeply yellow. The bill and the tarsus were black. Its biology is not well-studied.
Bishop's 'o'o - It was discovered in 1892 by Henry C. Palmer, a bird collector for Lord Rothschild. Its length was about 29 centimetres. The tail had reached a length of 10 centimetres. The plumage was general glossy black with yellow feather tufts on the maxillaries, beneath the wings and the undertail coverts. Their songs were simple two notes, took-took, Which could be heard for miles.
Kauai Oo - This bird was among the smallest of the Hawaiian honeyeaters, if not the smallest species, at just over 20 centimetres in length. It was black or very dark shiny brown with sparse yellow leg feathers and faint white banding on the breast and underwings. It was named the ʻōʻō ʻāʻā by the natives which means dwarf ʻōʻō. Like other honeyeaters it had a sharp, slightly curved bill for sampling nectar. Its favored nectar sources were Lobelia species and the ʻohiʻa lehua tree, and it also ate small invertebrates and fruit.
Hawaii Oo - The 'Hawaiʻi ʻŌʻō was first described by Blasius Merrem in 1786. It had an overall length of 32 centimetres , wing length of 11–11.5 centimetres , and tail length of up to 19 centimetres . The colour of its plumage was glossy black with a brown shading at the belly. It was further characterized by yellowish tufts at the axillaries. It had some yellowish plumes on its rump, but lacked yellow thigh feathers like the Molokaʻi ʻŌʻō, and also lacked the whitish edgings on its tail feathers like the Oʻahu ʻŌʻō. However it had the largest yellow plumes on its wings out of all the species of ʻŌʻō.