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Oswald is first seen at Naohs Base, where he is ordering Grigori soldiers to detain and test the Reborn for combat. As Ruca Milda, Illia Animi, and Spada Belforma are being pitted against strange enemies who began as human then morph into monsters, Oswald explains that, once a Reborn's memories have returned, their powers awaken. As the trio continues to fight, Oswald distinguishes their usefulness in war. He then sends them to the Western Battlefield in order to fight against Garam.
Oswald is later shown in one of Regnum's privy council meetings, in which he attempts to cover the incident at Naohs Base involving Ange Serena's escape as a minor experimental accident. As a high military official in the Regnum military, Oswald is occasionally mentioned during these meetings and is later revealed to be Hasta Ekstermi's employer. In one privy council scene, Oswald reveals the truth behind humans' ancestry, which spurs the council members into rejecting earthly powers, including the Church, in favor of obtaining the Manifest and returning to heaven.
Oswald's final meeting with the privy council reveals his plans to travel to the Tower of Dawn in order to obtain the Manifest while allowing Hasta to slaughter the council members so that they do not interfere. At the tower, Oswald confronts the group in a military robot named "Gigantess Omega", which is powered by the deceased god Gardle. After telling them that he plans to obtain the Manifest and use it to rule the world as god, the group engages him in battle and defeats him. After the fight, the robotic machine malfunctions and explodes, killing Oswald.