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 preventing the Alaafin from being an autocrat; the Oyo Mesi compelled many Afaafin to commit suicide during the 17th and 18th centuries. The head of the council of Oyo Mesi, the Bashorun, consulted the Ifa oracle for approval from the gods. New alaafins of Oyo were seen as appointed by the gods. They were regarded as "Ekeji Orisa," meaning "companion of the gods." The Bashorun had final say on the nomination of the new Alaafin, his power rivaling the king himself. For example, the Bashorun orchestrated many religious festivals; in addition to being commander-in-chief of the army, which gave him