Cushitic-speaking pastoralists organised by the gadaa generation-grade system who, from the 1520s, expanded out of a southern homeland at Harroo Walabuu across Bali, Dawaro, Shewa and beyond, in the vacuum left by the Adal–Ethiopian wars. By the 18th century Oromo groups held much of central, western and eastern Ethiopia; the Macha of the Gibe valley built the kingdoms of Jimma, Limmu-Enarya, Guma, Gomma and Gera before Menelik II's armies annexed them in the 1880s.
Not an ending: the Oromo are the largest nation of Ethiopia and hold the Oromia region. The map hands off to the Ethiopian Empire, which absorbed the Oromo lands during Menelik II's conquests of the 1880s–1890s.
The standard scholarly history of Oromo migration, gadaa and the Gibe states, in Cambridge's African Studies series; full text readable on the Internet Archive and verified via Open Library (OL3089088W, ISBN 9780521380119).
The same author's later study covering exactly the expansion era and its relations with the Solomonic state; verified via Open Library (OL21106358W, ISBN 1782045805).