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 despite its violent creation, it was held together by mutual self-interest. The government was able to provide unity for a vast area through a combination of local autonomy and imperial authority. Unlike the great savannah empires, of which Oyo may not be called a successor since it was a successor of Ife, there was little if any Muslim influence in the empire. It is known that at least some Muslim officials were kept in Metropolitan Oyo, and men capable of writing and calculating in Arabic were reported by French traders in 1787. Many believe the decline of the Oyo empire