Groves (2011) recognizes six Oryx species. The East African oryxes have traditionally been treated as a single species, Oryx beisa (and often even considered conspecific with the Gemsbok, O. gazella, of southwest Africa). According to Groves (2011), however, although they are very similar in appearance they are best treated as three distinct species: Beisa Oryx (O. beisa), found in northern and central Somalia and the Ogaden region of Ethiopia north to Berbera, west to Eitrea, and south into the Awash Valley; Galla Oryx (O. gallarum), of northern Kenya and northeastern Uganda extending into Somalia and southeastern Ethiopia; and Fringe-eared Oryx (O. callotis), found in southeastern Kenya and northeastern Tanzania. The fourth oryx species still living in the wild is the Gemsbok.
In addition to these four species are two species that went extinct in the wild. The Scimitar-horned Oryx (Oryx dammah), both sexes of which have long, slender, hollow horns that are annulated (i.e. with ring-like divisions) for the basal third and curve over the back, used to be found on the southern and northern edges of the Sahara Desert. They did not inhabit the desert proper, as does the Addax (Addax nasomaculatus). The former range experiences prolonged droughts, the most recent extending from the 1960s to the early 1990s! The southward spread of the Sahara Desert likely contributed to the sad decline of this species. Whe sedentary, herds consisted of 10 to 30 or even 100 individuals. During migration, groups of 1000 or more would aggregate. It is estimated the the wild population once numbered around a million individuals. The main causes of extinction were human population growth, motorized access to the desert,overhunting, and increased use of key habitats by livestock. The last known wild individuals were in Chad and Niger in the 1980s. Fortunately, captive populations were established beginning in the 1960s (around 4,000 captive animals are in the United Arab Emirates in a private collection and around 2,000 on private ranches in Texas, U.S.A.), so reintroduction is a possibility that is being actively pursued.
The only native oryx species outside Africa is the Arabian Oryx (Oryx leucoryx). This species was extinct in the wild by 1972, but free-ranging populations have been established in Israel, Oman, and Saudi Arabia. This species was formerly present throughout the Arabian Peninsula, extending north to Israel, Jordan, Kuwait, Iraq, Syria, and Sinai. Paching and overhunting in Oman eliminated the last wild individuals. Fortunately, captive breeding efforts had begun in the 1950s and reintroduction efforts began in the early 1980s and are ongoing. The world captive population is around 6,000 tp 7,000. The re-introduced free-ranging populations include only around 250 mature individuals,