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 about him, from his war against Aytolis, Oskar is proven to be willing to burn bridges with ally nations in order to achieve his ends, even resorting to thievery, kidnapping, and murdering. 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, not even noticing Darios' drastic personality change under possession of Velezark. In addition, Oskar is unloving, not even concerned for his son's health, and his final words were reprimanding his son.
—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.