Here's a snippet that I got from wikipedia.
"Another Sango pataki from Yorubaland goes like this; Sango was an Alaafin, ruling as king in Oyo. He learnt the some of the secrets of his special abilities from the Ibariba, his mother's people. He came back to Oyo and amazed with his ability to make fire come out of his mouth. He thus ruled with fear of his special powers. He had two war generals, Timi and Gbonka. Timi Agbale Olofa-ina could shoot arrows of fire. Gbonka was equally powerful. Alaafin Sango sensed that he was not safe with these two powerful generals and tried to set them against each other. He sent Gbonka to Ede, another town in Yorubaland, to capture Timi. Gbonka was immune to Timi's fire arrows, because he also mastered the secrets of fire, and put Timi to sleep by chanting incantations. He brought Timi back to Oyo. Sango insisted they fight again in the public square. Gbonka repeated his feat, was again victorious and cut Timi's head off. Gbonka then asked to be burned alive. He was burnt to ashes, and miraculously re-appeared on the third day. Gbonka then gave Sango the ultimatum to leave town for his infidelity. Sango is then said to have sadly left town and committed suicide on the Ayan tree in a place called Koso. His followers quickly rallied and declared defiantly that the king did not commit suicide, rendered in the Yoruba language as "Oba ko so". They then attacked anybody who said otherwise with the lightning that they had been allowed to wield by the death of their lord. That is why anyone killed by lightning in Yorubaland is buried by the members of the Royal Cult of Sango, whose members are typically referred to as Baba-mogba. As a tacit acknowledgement of the propaganda that is ascribed to the cultists, one of the late king's praise names is Olukoso- the one who did not hang."
After reading the whole thing and other sources, it sounds like this Olukoso was an old king of the Oyo empire that was said to have powerful abilities, specifically with fire, if I read it right.