Oliver sided with Lucifer in the rebellion against Heaven. After the war Oliver, Lucifer, and others were banished to Hell. He is one of the Hell-Lords, but was given punishment of humanity due to his pride and ambition by the other Hell-Lords out of fear.   Unfortunately for the Lords of Hell Olivier was born human into a mob family.   Upon killing someone for the first time Olivier regained his memories and planned his revenge against the rulers of hell and the eventual conquest of their realms.   To do so he would need an army which he didn't have so using magic he found Frank Castle and used magic to send those who frank killed to his own little realm of hell creating an army.  

He further helped create his army by arranging for Castle's family to be executed by the mob which had Castle going after the mafia.   This ensured the souls Olivier needed to build his army that would allow him to conquer the different realms in hell.   Olivier than began to feed on the Grigori stealing their divine essence to empower himself to further his plans of conquering all of hell.