Oscha is one of the four Lords of Gaudium, the high-ranking servants of the otherworld's ruler, Earl Tyrant, and he serves as the earl's closest adviser and the overseer of Chaos's growth. His name, sometimes romanized as Oscar or Osuka, is notable because its unusual spelling forms an anagram of the word Chaos. Among the four lords he is regarded as the calmest and the most mysterious.

His appearance is that of a jester-like living shadow. He wears a full-length robe of black fabric decorated with yellow, red, and green lines and patterns, and his face is hidden behind a simple white mask with long horizontal openings for the eyes and mouth, which gives him an eerie presence. On rare close-ups, a bloodshot eye and bluish, lifeless-looking skin can be glimpsed beneath the mask. His body is unnaturally flexible, and he tends to writhe and twist while speaking, often punctuating his words with a high-pitched, theatrical laugh. In many scenes he appears as a near-formless shadow with no visible legs, and arms that only sometimes emerge from his body, reflecting his ability to transform into and move as darkness.

In personality, Oscha is cold, cruel, and thoroughly manipulative. He is unfailingly polite in his speech, addressing others with honorifics and formal courtesy, but his manner is closer to insolent condescension, and he often makes veiled taunts, such as provoking the warrior Kaze about his attachment to the swordsman Makenshi. He rarely takes direct action and prefers to advance his schemes through other people, whether allies or enemies. His single apparent passion is the nourishment and expansion of Chaos, and he becomes noticeably animated, almost delighted, whenever the subject arises. He is also a patient, long-term strategist, able to plan on a scale that spans the entire story, and he recognized the unusual nature of the twins Ai and Yu from the beginning and began steering them as fuel for Chaos long before others understood their significance.

His role in the story is largely that of the earl's right hand. He attends to Earl Tyrant like a devoted servant, serving his meals, observing the state of Chaos, and reporting on it, though he is also the only lord who can make the others look foolish before the earl and is entrusted with quietly disposing of useless underlings. He rarely confronts the protagonists directly, and his most notable direct encounter with Kaze involves hiding on a factory ceiling while concealing his presence and surviving a shotgun blast to the torso, with the pellets falling away without piercing him. Most of his influence is exerted through his creations and inventions. He created the living puppet Crux, a small fairy-like being used for tracking and observation, and he produced a new version of her in later events. He revived the dead magician Red Mist, the elder brother of Makenshi, by sealing his soul into a straw doll with his jealousy of his younger brother deliberately amplified, so that the ensuing battle would feed Chaos with the swordsman's grief. He also modified the otherworld clock Isabel as a trap for the companions, built the automatic searching device Walnut and gave it to the lord Helba for locating fragments of Omega, and commanded various puppet-type monsters. Late in the series he reveals that the twins were born from Chaos in the aftermath of an event that sent their adoptive parents into Wonderland. In the final episode he is caught in the shockwave of a clash between two great beasts, loses part of his mask, and falls into the vortex of Chaos, leaving the story with his nature still unexplained. In the later continuation of the story, set in the outer world, he survives and continues his servant-like duties, now attending Helba after her revival as the self-proclaimed queen of the otherworld, while quietly creating a second Crux to search for the missing Makenshi.

His key relationships center on Earl Tyrant, whom he serves with complete loyalty as adviser, attendant, and observer of Chaos, enduring the earl's tantrums and thrown cutlery without losing his composure. He treats the other lords with exaggerated deference, yet he is trusted enough to manage the earl's household and to dispose of those deemed useless, and he maintains a cooperative working relationship with Helba, to whom he provides technical support. His connection to his puppet Crux is that of creator and tool, although Crux eventually displays signs of independence and attachment to Makenshi. Against the heroes he functions as a persistent antagonist, and through his revival of Red Mist he becomes the indirect cause of deep grief for Makenshi.

Oscha shows little visible change or development, and his true nature remains deliberately ambiguous. Several hints in the story suggest that he may himself be an embodiment of Chaos, a counterpart to the earl who merged with the same force, but this is never confirmed. In the continuation material he appears with his mask still damaged, his pale and lifeless skin and unsettling eye exposed, still carrying out household tasks while pursuing his own quiet agenda, reinforcing the impression of an enduring, patient presence devoted to Chaos.

His abilities reflect his role as a schemer and technician more than a warrior. He is a master of puppet arts, able to create artificial life such as Crux and to grant temporary life to the dead by implanting souls into straw dolls. He can shift his body into a shadow, move through concealment, and erase his presence so completely that even Kaze cannot detect him. His body has shown remarkable durability, withstanding direct gunfire without harm. He is also a skilled inventor and engineer, creating tracking devices, traps, and search mechanisms, and he commands an arsenal of puppet-type monsters in combat. Above all, his greatest ability is his intellect, allowing him to manipulate events across the entire course of the story in service of the growth of Chaos.