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 After this, Oba Orompoto led attacks to obliterate the Nupe to ensure Oyo was never threatened by them again. During the reign of Oba, Ajiboyede, he held the first Bere festival, an event to celebrate peace in the kingdom. Celebrated regularly, it would retain much significance among the Yoruba long after the fall of Oyo. Under his successor, Abipa, the Yoruba repopulated Oyo-Ile and rebuilt the original capital. Despite a failed attempt to conquer the Benin Empire sometime between 1578 and 1608, Oyo continued to expand. The Yoruba allowed autonomy to the southeast of metropolitan Oyo, where the non-Yoruba areas