Oslo El Gustave rose to the rank of duke in the Freyjagard Empire solely through his formidable combat prowess. His rule became infamous for the extreme "beautification law," which outlawed all agriculture and productive labor within his territory. This policy, born from an obsessive devotion to Emperor Lindworm von Freyjagard – whom Oslo viewed as the world's sole legitimate ruler – triggered widespread starvation, forcing desperate citizens into cannibalism. Oslo interpreted their suffering and deaths as a tribute to the emperor, a delusion central to his vision of a "beautiful" domain.

This descent into tyranny originated in a pivotal past event. Initially, Oslo collaborated with his childhood friend, Count Blumhardt, aiming to reform the empire's oppressive conditions, including a failed assassination attempt against Emperor Lindworm. Captured and released, they witnessed Lindworm declare himself an immortal, god-like ruler destined for world domination. This encounter shattered Oslo's former ideals, warping his loyalty into fanatical obsession.

Oslo wielded significant combat power, specializing in fire-based magic. His signature technique, "Heaven's Flame" (Rage Soleil), involved hurling a spear imbued with fire magic. Upon impact, it ignited an inextinguishable inferno consuming everything nearby; water proved useless, requiring the destruction of the spear catalyst itself. Emperor Lindworm once deflected this attack with a single sword strike, while Aoi Ichijō, one of the seven prodigies, later neutralized it during the defense of Dormundt by redirecting an active missile into the flames.

His reign culminated in confrontation with the series' protagonists and allies. The Azure Brigade, a faction of reformist nobles including the knight Jeanne du Leblanc, opposed his cruelty. Shinobu Sarutobi, a prodigy journalist, aided Jeanne in exposing Oslo's atrocities. His downfall occurred when Tsukasa Mikogami, leader of the prodigies, orchestrated a strategy to minimize casualties during Oslo's premature deployment of Heaven's Flame against Marquis Findolph's territory, highlighting Oslo's strategic ineptitude amid his territory's collapse.

Throughout his arc, Oslo remained defined by egomania, delusions of grandeur, and ruthless disregard for human life. His actions exemplified the corruption festering within the Freyjagard Empire's nobility, serving as a catalyst for broader conflicts involving the seven prodigies.