—Oskar to Darios and his first words on screen.
Although never fought by the player, during his conquest of the Aytolis kingdom, Oskar gains the assistance of Validar, Iago and Gharnef, who lead his kingdom's forces into battle during several chapters in the game. Oskar's ultimate goal, however, is to obtain the Shield of Flames from Aytolis and utilize its power to revive the Chaos Dragon, Velezark, and control the world alongside it.
After the spirit of Velezark possesses the body of Darios, who at the time was in possession of the Shield of Flames, Oskar takes residence in Aytolis Castle, where preparations are made for Velezark's physical resurrection. The ritual requires a sacrifice of royal blood, and so he and the possessed Darios intend to sacrifice Aytolis's Queen Yelena. However, their plan is foiled by Yelena's children, Rowan and Lianna, and the heroes they had gathered.
Unable to sacrifice Yelena, the possessed Darios turns on Oskar and impales him with his sword, using the king's royal blood to revive the Chaos Dragon’s full power.
While little is said or even seen about Oskar, from his war against Aytolis, it could be said Oskar is proven to be willing to burn bridges with ally nations in order to achieve his ends, as he turned on Aytolis despite years of peace between it and Gristonne.
The waiting screens suggest that he was an absent father in Darios' life, which possibly made it easier for the latter to turn on him before becoming possessed. Oskar appears to be very neglectful or oblivious, not even noticing Darios' drastic personality change under possession of Velezark and simply assumed his son had come back to him. When a possessed Darios stabbed him, Oskar was shocked and stared at his son with betrayal and sadness, his last words being a reprimand of his son for his apparent treachery.
—Oskar talking to Darios about Rowan and Lianna following him.
—Oskar's final words
- Oskar shares his English voice actor, DC Douglas, with Wrys and Barst in Fire Emblem Heroes as well as Gunter from both Fire Emblem Fates and Heroes.
- Despite being ostensibly the main antagonist of the game, Oskar has has less then two minutes of screen time, has only two lines within the game and is immediately killed without a struggle in the very chapter he first appears.
- Due to not being a fightable character, Oskar also has no appearance within the game's adventure mode.