We don't do history so manage this
Oyo Empire was the most politically organized entity founded by the Yoruba speaking people in the present day Nigeria. The empire was well organized and was influential and powerful. It at a time controlled the politics and commerce of the area known today as a southwestern Nigeria.
The political administration of Oyo Empire was based on complex system of checks and balances involving layer and structure of traditional rulers. At the center of the political structure is the Alaafin who was the king. Most powers were controlled by him. He wielded both political and spiritual powers. Under the administrative structure of the Alaafin was the Aremo who was always the first son of the Alaafin. In the early stage, if an Alaafin dies the Aremo takes over but it was found that some Aremo killed their father so as to rise to the throne. Later a decree was made demanding that the Aremo should commit suicide upon the death of an Alaafin. His administration was headed by three lieutenants namely the Ona Efa who was the Empires Chief Justice, the Otun Efa who was in charge of the Sango shrine and the Osi Efa who was the controller of he palace finances.
The next political structure was the Oyomesi. The Oyomesi was a council of prominent chiefs of the Empire numbering about six or seven sometimes. The Bashorun was the title of the head of the Oyomesi. The main function of the Oyomesi was to act as a check to the excessive powers of the Alaafin. The selection and dethronement of Alaafin was the exclusive powers of the Oyomesi. In a situation were the Oyomesi losses confidence on an Alaafin, the Bashorun is expected to present that Alaafin with a Calabash which signifies that the Oyomesi, ancestors, people have lost confidence on him. The Alaafin is then expected to commit suicide.
Another important political structure was the Ogboni headed by the Olowu. The Ogboni was made up of representatives of various lineages. The Ogboni acted as checks to the excessive power of the Bashorun. For someone to be a Bashorun, the Ogboni must give their own approval. In some cases the Alaafin made use of Olowu to check the unnecessary presentations of power by the Bashorun led Oyomesi.
Next in line was the army leaders headed by the Are-Ona-kakanfo that was the army commander. The Are-Ona-Kakanfo was responsible to the Alaafin and the Bashorun. They army officers were appointed by the Alaafin but their promotions were carried out by the Oyomesi. Are-Ona-Kakanfo was not expected to loose war. If he loses a war, he is expected to commit suicide or go into exile. This made most of the people that served the Empire as Are-Ona-Kakanfo to develop themselves into great warriors.
Other Yoruba areas that formed part of the empire were ruled by their Oba, council of Elders or the Ogboni confraternity. Each of these communities had an Alaafin supervisor resident in it known as Ajale. Because of communication gap Ajale enjoyed relative freedom making some of them corrupt. In order to check this, the Alaafin established very special messengers known as Ilari. Their role was mainly to supervise different Ajale under their jurisdiction. Some Ilari became corrupt and very oppressive.