Oyo Empire metropolitan Oyo. This may have been fine in the 15th century when Oyo controlled only its heartland. But to make and maintain farther conquest, the structure underwent several changes. Oyo maintained a semi-standing army of specialist cavalry soldiers called the "Eso", the "Esho" or, formally, the Eso of Ikoyi. These were 70 junior war chiefs who were nominated by the Oyo Mesi and confirmed by the Alaafin of Oyo. The Eso were appointed for their military skill without regard to heritage and were led by the Are-Ona-Kakanfo. After Oyo's return from exile, the post of Are-Ona-Kakanfo was established as the
Oyo Empire the Alaafin of Oyo was supreme overlord of the people, he was not without checks on his power. The Oyo Mesi and the Yoruba Earth cult known as Ogboni kept the Oba's power in check. The Oyo Mesi spoke for the politicians while The Ogboni spoke for the people backed by the power of religion. The power of the Alaafin of Oyo in relation to the Oyo Mesi and Ogboni depended on his personal character and political shrewdness. The Oyo Mesi were seven principal councilors of the state. They constituted the Electoral Council and possessed legislative powers, similar to today's