The origins of the Oyo Empire lie with Oranyan, the last prince, grandson of the Oduduwa.
Oranyan made an agreement with his brother to launch a punitive raid on their northern neighbors for
insulting their father Oduduwa , the first Ooni of Ife . On the way to the battle, the brothers quarreled and the army splitup. Oranyan’s force was too small to make a successful attack, so he wandered the southern shore until reaching Bussa.
There the local chief entertained him and provided a large snake with a magic charm attached to its throat.
The chief instructed Oranyan to follow the snake until it stopped somewhere for seven days and disappeared into the ground. Oranyan followed the advice and founded Oyo where the serpent stopped. The site is remembered as Ajaka . Oranyan made Oyo his new kingdom and became the first “Oba” (meaning ‘king’ or ‘ruler’ in the Yoruba language ) with the title of “Alaafin of Oyo” (Alaafin means ‘owner of the Palace’ in Yoruba).
At a time, Oyo-ile was at war with the Bariba of Borgu who wanted to subjugate the new City still under construction. Orangun Ajagunla of Ila, Oranmiyan’s elder brother stormed in with his men to assist. Not long after the war was won, Ora nyan welcomed a son Ajuwon Ajaka, much later Arabambi (Sango)was born by the woman from Tapa (Nupe ), It is believed that the name “Sango” was given by his maternal grandfather or He adopted it from the local name for the God of Thunder, Either way the royal family was devoted to The Spirits of Thunder(Jakuta) and War(Ogun). Early period (14th century–1535).
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, Sango , who was later deified as the deity of thunder and lightning. Ajaka was restored after Sango’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.
You can read more from the book of Samuel Johnson. “The History of the Yorubas: From the Earliest Times to the Beginning of the British Protectorate”.