One of the Kunitsukami's in Japanese mythology. He is the grandfather of Susano-o's wife, Kushinada-Hime.
He is known as the god of the mountains but is also considered to be the god of water and rice fields because he ruled over water sources and harvests.
Oyamatsumi can be found in a shrine at Old Kōjimachi, where he has gotten lost and cannot find his way out. Upon meeting Aleph, he joins his party, hoping to find a way out. After the group exits the shrine, he grants them the Right Leg of Masakado as thanks.