The Oyo Empire, like many empires before it, used both local and tributary forces to expand its domains. The structure of the Oyo military prior to its imperial period was simple and closer aligned to the central government in 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 or Esho . 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 Aare-Ona-Kakanfo. After Oyo’s return from exile, the post of Aare-Ona-Kakanfo was established as the supreme military commander. He was required to live in a frontier province of great importance to keep an eye on the enemy and to keep him from usurping the government. During Oyo’s imperial period, the Aare-Ona-Kakanfo personally commanded the army in the field on all campaigns.
Since the Are-Ona-Kakanfo could not reside near the capital, arrangements had to be made for the latter’s protection in case of emergency. Forces inside
metropolitan Oyo were commanded by the Bashorun, leading member of the Oyo Mesi. As stated earlier, Metropolitan Oyo was divided into six provinces divided evenly by a river. Provincial forces were thus grouped into two armies, under the Onikoyi and the Okere for the east
and west side of the river respectively. Lesser war chiefs were known as Balogun, a title carried on by the soldiers of Oyo’s successor state, Ibadan.
Tributary leaders and provincial governors were responsible for collecting tribute and contributing troops under local generalship to the imperial army in times of emergency. Occasionally, tributary leaders would be ordered to attack neighbors even without the backing of the main imperial army. These forces were often utilized in Oyo’s distant campaigns on the coast or against other states.