The Oyo Empire is an empire based around the capital city of Oyo-Ile, established in 1400 CE. Oranyan, the first oba (king) of Oyo, was succeeded by Oba Ajaka, Alaafin of Oyo. Ajaka was deposed, because he lacked Yoruba military virtue and allowed his sub-chiefs too much independence. Leadership was then conferred upon Ajaka's brother, Shango, who was later deified as the deity of thunder and lightning. Ajaka was restored after Shango's death. Ajaka returned to the throne thoroughly more warlike and oppressive. His successor, Kori, managed to conquer the rest of what later historians would refer to as metropolitan Oyo in 1424.
The heart of metropolitan Oyo is its capital at Oyo-Ile. The two most important structures in Oyo-Ile are the 'afin' or palace of the Oba and his market. The palace is at the center of the city close to the Oba's market called 'Oja-oba'. Around the capital is a wall for defense with 17 gates. The importance of the two large structures (the palace and the Oja Oba) signified the importance of the king in Oyo.
The Oyo Empire is not a hereditary monarchy, nor an absolute one. The Alaafin of Oyo is carefully selected by the Oyo Mesi and is not always directly related to his predecessor, though he does have to be descended from Oranyan (also known as Oranmiyan), a son of Oduduwa (also known as Odudua, Odua ) and to hail from the Ona Isokun ward (which is one of the three royal wards). At the beginning of the Oyo Empire it was usually the Alaafin's oldest son that succeeded his father to the throne. However, this sometimes led to the oldest son i.e the first born prince, the Aremo, hastening the death of his father. Independently of the possible succession to his father, the Aremo was quite powerful in his own right. For instance, by custom the Alaafin abstain from leaving the palace, except during the important festivals, which curtailed his power in practice. By contrast, the Aremo often left the palace. This led noted historian Johnson to observe: "The father is the king of the palace, and the son the King for the general public". The two councils which check the Alaafin have a tendency to select a weak Alaafin after the reign of a strong one in an effort to keep the office from becoming too powerful.
Certain religious and government officials, usually eunuchs, are appointed by the Alaafin of Oyo. These officials are known as the ilari or half-heads because of the custom of shaving half of their heads and applying what was believed to be a magical substance into it. There were hundreds of Ilari divided evenly among the sexes. Junior members of the Ilari do menial tasks while seniors act as guards or sometimes messengers to the other world via sacrifice. They have titles referencing the king such as oba l'olu ("the king is supreme") or madarikan ("do not oppose him"). They also carry fans of green or red as credentials.
All sub-courts of Oyo have Ilari who act as both spies and taxmen. Oyo appoint them to visit and sometimes reside in the Foreigners Corridor to collect taxes and record on foreign military successes so that the Alaafin of Oyo can receive proper benefits.
While the Alaafin of Oyo is supreme overlord of the people, he is not without checks on his power. The Oyo Mesi speak for the politicians. The power of the Alaafin of Oyo in relation to the Oyo Mesi and Ogboni depends on his personal character and political shrewdness.