The Oromo people (Oromo: Oromoo; Ge'ez: 'oromo; 'Oromo) are an ethnic group inhabiting Ethiopia and parts of Kenya and Somalia. They are the largest ethnic group in Ethiopia and the wider Horn of Africa. According to the 2007 census, they represent approximately 34.5% of Ethiopia's population, while others estimate they make up about 40% of the population. With the total Ethiopian population thought to be over 102 million, the number of Oromo people exceeds 35 million in Ethiopia alone.

Historically, Afaan Oromo-speaking people used their own Gadaa system of governance. Oromos also had a number of independent kingdoms, which they shared with the Sidama people. Among these were the Gibe region kingdoms of Gera, Gomma, Garo, Gumma, Jimma, Leeqa-Nekemte and Limmu-Ennarea.  The earliest known documented and detailed history of the Oromo people was by the Ethiopian monk Abba Bahrey who wrote Zenahu le Galla in 1593, though the synonymous term Gallas was mentioned in maps or elsewhere much earlier. After the 16th century, they are mentioned more often, such as in the records left by Abba Pawlos, Joao Bermudes, Jerorimo Lobo, Galawdewos, Sarsa Dengel and others. These records suggest that the Oromo were pastoral people in their history, who stayed together. Their animal herds began to expand rapidly and they needed more grazing lands. They began migrating, not together, but after separating. They lacked kings, and had elected leaders called luba based on a gada system of government instead. By the late 16th century, two major Oromo confederations emerged: Afre and Sadaqa, which respectively refer to four and three in their language, with Afre emerging from four older clans, and Sadaqa out of three. These Oromo confederations were originally based on southern parts of Ethiopia, but started moving north in the 16th century in what is termed as the "Great Oromo Migration".  According to Richard Pankhurst, an Ethiopia historian, this migration is linked to the first incursions into inland Horn of Africa by Imam Ahmad ibn Ibrahim. According to historian Marianne Bechhaus-Gerst, the migration was one of the consequences of fierce wars of attrition between Christian and Muslim armies in the Horn of Africa region in the 15th and 16th century which killed a lot of people and depopulated the regions near the Galla lands, but also probably a result of droughts in their traditional homelands. Further, they acquired horses and their gada system helped coordinate well equipped Oromo warriors who enabled fellow Oromos to advance and settle into newer regions starting in the 1520s. This expansion continued through the 17th century.  Both peaceful integration and violent competition between Oromos and other neighboring ethnicities such as the Amhara, Sidama, Afar and the Somali affected politics within the Oromo community. Between 1500 and 1800, there were waves of wars and struggle between highland Christians, coastal Muslim and polytheist population in the Horn of Africa. This caused major redistribution of populations. The northern, eastern and western movement of the Oromos from the south around 1535 mirrored the large-scale expansion by Somalis inland. The 1500-1800 period also saw relocation of the Amhara people, and helped influence contemporary ethnic politics in Ethiopia.  According to oral and literary evidence, Borana Oromo clan and Garre Somali clan mutually victimized each other in seventeenth and eighteenth centuries, particularly near their eastern borders. There were also periods of relative peace. According to Gunther Schlee, the Garre Somali clan replaced the Borana Oromo clan as the dominant ethnic group in this region. The Borana violence against their neighbors, states Schlee, was unusual and unlike their behavior inside their community where violence was considered deviant.

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