Oma'os are the most common of the Hawaiian thrushes, a group whose members once occurred on each of the six major Hawaiian islands. Found only on the island of Hawaii, Oma'os are medium-sized, fruit-eating birds that occur primarily in high-elevation native rainforests. Despite healthy population numbers, only 30% of this species' former range remains intact, and Oma'os face the same threats that have decimated other native Hawaiian bird populations (habitat destruction, introduced predators, and diseases).

Adult Oma'os are rather drab, non-descript solitaires. Birds are gray-brown above and lighter gray below, with a short, dark bill, a dark eye, and dark legs. As a member of the thrush family, Oma'os have a juvenile plumage pattern with heavy scalloping on the breast. The breast on juvenile birds is buffy-whitish in color, with black scalloped markings. Young Oma'os also have browner backs than do adults. Because of the inconspicuous appearance of Oma'os, birds are often first located by voice. One typical call is a sharp rasping note, while another sounds like a shrill police whistle. The song of Oma'os is a distinctive mixture of chirps and whistles.

Oma'os are currently found only on the eastern and southern slopes of the island of Hawaii. Two main populations are known from wet rainforest habitats in Ka'u and Hamakua-Puna; a small, separate population is found in alpine scrub habitat on the slopes of Mauna Loa. Oma'o numbers appear to be stable in areas with high-quality habitat, and populations might be increasing in sites below 1,200 meters of elevation.

Oma'os occur mainly in native 'ohi'a and mixed 'ohi'a /koa forests above 1,000 meters. Forests preferred by Oma'os typically have a closed canopy 25-40 meters in height, and numerous fruiting trees in the understory. A small population of Oma'os is also found above treeline in alpine scrub on Mauna Loa volcano. In forested habitat, Oma'os are primarily frugivorous, feeding on fruits from understory trees and shrubs. Birds feed on a wide variety of food sources, including berries from both native and non-native plants. Oma'os also feed on a number of different invertebrates, preying mainly on caterpillars and spiders.

Oma'os are usually solitary, but individuals can be found in pairs throughout the year, with pair bonds lasting at least one breeding season. Courtship behavior is most often seen between January and March, with most breeding taking place between April and August. Females are responsible for both nest construction and incubation of one or two eggs. Incubation lasts for about 16 days, and the young remain in the nest for about 19 days before fledging. Both sexes feed nestlings, and both adults provide parental care for more than three weeks after young birds leave the nest.

Evidence exists that Oma'os have developed partial resistance to, or at least tolerance for, current strains of avian poxvirus and malaria. This may account to some degree for this species' continued persistence where other species have disappeared. Despite this suspected tolerance, Oma'os still fare best above 1,500 meters, where disease-carrying mosquitoes are uncommon. The activities of feral pigs in native forests not only result in the devastation of native understory plants, they also create favorable conditions for mosquito breeding. The introduction of cold-tolerant mosquitoes, along with new avian diseases, is a potential major threat to Oma'os and other forest-dwelling Hawaiian birds. The presence in native forests of non-native terrestrial predators, such as rats and feral cats, represents a continuing threat to Oma'os and other species.

Hakalau Forest National Wildlife Refuge was created in 1985 to protect Oma'os, other native Hawaiian forest birds, and their habitats. A cooperative strategy of purchasing, restoring and managing forested lands above 1,500 meters continues to play an important role in the conservation of this species. These efforts ensure that the species remains common in numerous parks, wildlife refuges, and natural area reserves. Aggressive efforts to control and fence out feral pigs, goats, cattle, and sheep have allowed native vegetation to return in some areas but there is no good evidence yet that this has resulted in increased bird numbers. Invasive weeds and rats continue to be a major and unresolved problem. Since other Hawaiian thrushes were little studied before they disappeared (either being extinct or presumed extinct), much of what we know about these birds comes from recent studies of the Oma'o life history and populations. Future conservation efforts of rediscovered thrush populations would likely be based on study of the Oma'o.