Oscar Orcus co-founded the Liberators, resisting the god Ehit and the Holy Church's tyranny in ancient Tortus. He designed and constructed the Great Orcus Labyrinth to safeguard ancient knowledge and test future challengers capable of confronting the gods. His background traces to a Velka Kingdom orphanage, where he learned cooking and cleaning while caring for younger children. Adoption by Karg Orcus led him to inherit the Orcus Workshop title; he refused weapon commissions to avoid authorities. This peaceful existence shattered when the Holy Church, aided by associate Ping Warress, kidnapped his adoptive siblings, compelling him to abandon reluctance and join Miledi Reisen's rebellion.

Appearance-wise, Oscar possessed a slender build, shoulder-length black hair usually tied in a ponytail, blue eyes, and black-rimmed glasses. His attire featured a black coat, vest, pants, boots, and a white dress shirt with a neck scarf, often accessorized with a black umbrella—all potent artifacts. Following reincarnation after Miledi's death and Ehit's defeat, he became part of another world's Cleyer noble family under the name Weiss, now with blond hair and blue eyes while retaining past memories.

Personality blended gentlemanly conduct with fierce protectiveness toward family. Initially avoiding confrontation to shield loved ones from Church scrutiny, their endangerment transformed him into a resolute fighter. He prioritized logic over faith, though later explored concepts like reincarnation when facing permanent separation from comrades. Despite a calm demeanor, he exhibited volatile anger, especially toward Miledi, engaging in frequent arguments. He harbored a notable fetish for maids and judged their attire critically. Professionally, he took immense pride in his artifacts, frequently incorporating impractical or hazardous features, and vehemently rejected being defined solely by his glasses. As a craftsman, he valued simplicity over ostentation and believed Synergist abilities enabled creating historically significant works.

Abilities centered on Creation Magic, an ancient art manipulating inorganic materials and infusing artifacts. As a Synergist, he mastered Transmutation and derivatives: Ore Appraisal, Precision Transmutation, and Perception for material analysis; Synthesis and Desynthesis for alloy creation/separation; Image Supplementation and Composition for imagination-based crafting; Accelerated, Automatic, and Compression Transmutation for efficiency; Decomposition for structural breakdown. Combat artifacts included: an umbrella channeling elemental spells (wind, fire, water, ice, earth, lightning); Metamorph Chains enhanced with gravity/spatial magic for restraint; Sable Gloves releasing gravity-controlled threads; Onyx Boots with wind magic for mobility; a Treasure Trove ring functioning as a hyperspace arsenal. His domain skill, Toy Box, transmuted areas into lethal, self-repairing traps. He demonstrated strategic brilliance, devising tactics like conjuring artificial meteors with ally Naiz.

Key relationships defined his arcs. Loyalty to Miledi evolved into undying devotion, epitomized by his declaration to follow her "even to hell and back," despite their belligerent rapport. With fellow Liberator Vandre Schnee, he shared a vitriolic friendship, crafting the sword Dragon Slayer specifically to counter him. His bond with Meiru involved an accidental marriage proposal during a plea to heal his siblings, later embraced by her pirate crew. The Liberators collectively experienced a Ghost Reunion Ending, reuniting in the afterlife after Miledi's soul passed on.

His legacy endured through the Orcus Labyrinth, where a recorded message revealed the gods' malevolence to challengers like Hajime Nagumo. Hajime inherited Oscar's Synergist legacy, utilizing his workshop and resources to forge weapons and prosthetics. Posthumously, Oscar's reincarnation as Weiss signified continuity beyond his original life.