Oumitsunu is a legendary king of Izumo mentioned in the Fudoki, a mythical record of his history of Japan. He was a demigod, being the grandson of Susano-O, and had the size and strength of a giant, which he used to pull land via ropes to expand his kingdom. His tale is similar to the Daidara-bocchi, also of Japanese lore, and the Polynesian Maui.
"The giant of the Izumo-no-kuni Fudoki, or the Records of Izumo. He performed the Kuni-biki, roping in land from the opposing Silla, in an effort to expand Izumo. All over Japan, there are stories of how Daidarabocchi giants shaped the land. Oumitsunu is thought to be the root of these tales."
—Shin Megami Tensei IV Apocalypse compendium
Oumitsunu can teach Nanashi the Berserker God, Tarukaja and Titanomachia skills through his Demon Whisper. He benefits from learning Physical attack skills.
Devil Summoner: Raidou Kuzunoha vs. The Soulless Army
"The giant of the Izumo-no-kuni Fudoki, or the Records of Izumo. Perfomed the Kuni-biki, roping in land from the opposing Silla, in an effort to expand Izumo."
—Devil Summoner: Raidou Kuzunoha vs. the Soulless Army Devil Chart
"The giant of the Izumo-no-kuni Fudoki, or the Records of Izumos. Performed the Kuni-biki, roping in land from the opposing Silla, in an effort to expand Izumo."
—Persona 3 Compendium
Devil Survivor 2
"The giant of the Izumo-no-kuni Fudoki, or the Records of Izumos. He performed the Kuni-biki, roping in land from the opposing Silla, in an effort to expand Izumo. All over Japan, there are stories of how Daidarabocchi giants shaped the land. Oumitsunu is thought to be the root of these tales."
—Devil Survivor 2 Compendium
Oumitsunu is unlocked by reaching Fate rank 5 with Jungo Torii.