There was total separation of powers as well as checks and balances in Oyo kingdom.
The centralized government of Oyo was headed by the Alaafin of Oyo assisted by Ona Efa (chief justice), Otun Efa (head of Sango deity) and the Osi Efa (finance minister).
Then there was a council of seven (Oyomesi) headed by the Bashorun, who checked the power of Alaafin. The Ogboni cult headed by the Olowo checked the powers the Oyomesi. Thus, the conquered villages were headed by the Ilaris.
However, Oyo had a strong army that was headed by Are-Ona-Kakanfo assisted by seventy war chiefs called Esos. And when they lost in any war; the Esos involved are banished and the Kakanfo forced to commit suicide.
Meanwhile, the Alaafiin’s first son that ruled with him was forced to commit suicide on the death of his father.