The Oromo, the largest single ethnic group in Ethiopia consisting of about 22 million people or roughly one third of the country's total population, are found mainly in southern and central Ethiopia. In the north there is a relatively small group living in Wollo. Large numbers of Oromo can also be found in neighboring Kenya. In spite of their large numbers they have never been dominant in Ethiopian society, mainly because of their internal diversity and geographical spread. In the Amhara language they are called the Galla. The Oromo were less united both religiously and socially than the Amhara and Tigrayans. Historically, they have never formed one Oromo state, but were organized into smaller units of clans or villages. The Oromo consisted of four main groups. First, the western Oromo, mainly in Wallaga, have largely been Christianized by the Protestant Mekane Yesus Church. Secondly, the northern Oromo of Mecha-Tulama, living in Shoa and the area to the south of it, are mostly Christians of the Ethiopian Orthodox church and have become more integrated into Amhara culture than the other Oromo groups. This group speaks Amhara as well as Oromifa (the Oromo language in the Eastern Kushitic group of languages). Thirdly, the southern Oromo of Arsi (or Aroussi) and Borana had a semi-nomadic life style. Some are Muslims and others belong to traditional religions.

Fourth, the eastern Oromo of Harage consists of the Muslim urban people of Harar and Dire Dawa and rural populations living around these towns and the areas to the west of them. This group traditionally linked itself to the Arab world. The lands of the Oromo are shared with many other ethnic groups who live among them, such as Gurage, Sidamo, and Somali. There have been attempts to establish an independent "Oromia" state through integration of various Oromo regional and religious groups into one Oromo state. However, drawing boundaries along ethnic lines in the south is extremely difficult since the southern populations are very mixed and their ethnic identification is impeded by vast tribal and religious differences. Many non-Oromo southerners resist Oromo domination and the boundary problems (caused by the aforesaid attempt to build an independent Oromia may well create conflicts in the future. Also, one of the important tasks of Oromo leaders is to unite Muslim and Christian populations of the four groups.

Oromos have a well-described view on our world, in which the highest force, called "Waaka", rules the kosmos. The creative power of Waaka is to be found in each living entity, be it human, animal, or plant. Nature and animals are treated gently and with respect, in order to establish the balance between the needs of all living species and the omnipresent difficult ecological conditions. He who uses his Waaka powers well and lives in peace with his environment will be rewarded with just the amount of rain he needs.

The traditional political system was based on "gada", a system of five age classes (so-called "luba"). All men wento through this cycle and were allowed some amount of decision power. A person would shift to the next luba every eight years. This system's main advantage was that personal ambitions were suppressed in favor of the people as a whole: political power was distributed among generations, not among individuals. Nowadays the gada still exists, but not to its original extent. In the beginning of the nineteenth century local clan leaders rapidly gained military power and control over trade routes; this fact gradually lead to a separate administrative and judicial system, which conflicted with gada and infected it. In the 1980s the remains of gada were directly attacked by the communists: the extent of the damage to the system is not entirely clear yet.