The Oromo people followed their traditional religion and used the gadaa system of governance. A leader elected by the gadaa system remains in power only for 8 years, with an election taking place at the end of those 8 years. From the 18th century to the 19th century, Oromos were the dominant influence in northern Ethiopia during the Zemene Mesafint period. Various current estimates state that probably around 70-81% of Oromos follow Islam, 29-30% follow Christianity, and fewer than 3% have retained their traditional beliefs. They have been one of the parties to historic migrations, and wars particularly with northern Christians and with southern and eastern Muslims, in the Horn of Africa.
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.
According to Francesca Declich, there was a flourishing market in Somalia for Oromo slaves in 19th and early 20th centuries, both in agricultural areas surrounding the Shebelle River, as well as in urban and pastoral settings. These Oromo slaves were either kidnapped from their home regions, captured in raids or were captives who had fought back. The records from the Somali markets, such as in Mogadishu, indicate that the highest price for slaves were paid for "Galla woman" in the 15–20 year age group meant to work as a concubine, while a Galla teenage for domestic work and girl child in the age group of 8–10 attracted much lower prices.
Barentu/Barentoo or (older) Baraytuma is the other moiety of the Oromo people. The Barentu Oromo inhabit the eastern parts of the Oromia Region in the Zones of Mirab Hararghe or West Hararghe, Arsi Zone, Bale Zone, Debub Mirab Shewa Zone or South West Shewa, Dire Dawa region, the Jijiga Zone of the Somali Region, Administrative Zone 3 of the Afar Region, Oromia Zone of the Amhara Region, and are also found in the Raya Azebo woreda in the Tigray Region.
Like other ethnic groups in the Horn of Africa and East Africa, Oromo people regionally developed social stratification consisting of four hierarchical strata. The highest strata were the nobles called the Borana, below them were the Gabbaro (some 17th to 19th century Ethiopian texts refer them as the dhalatta). Below these two upper castes were the despised castes of artisans, and at the lowest level were the slaves.