Oyo, also written as Oyou, is a character from the Umibōzu story arc of the anime Mononoke. She is the younger sister of the Buddhist monk Genkei, five years his junior, and the two grew up together in near isolation on a small island after their parents died at sea. Because they had only each other, the siblings developed an extremely close bond, one that Genkei later describes as excessively intimate.

Oyo is portrayed as a beautiful and deeply loving young woman who is devoted to her brother and willing to do anything for him. Her defining trait is this devotion, which is complicated by the fact that she and Genkei came to share romantic feelings for each other, feelings that were forbidden by their relationship. When the island's village, faced with ships sinking and being destroyed at sea, asked Genkei to become a human sacrifice by being sealed alive in a hollow boat and set adrift, Oyo intervened. The night before he was to enter the boat, she confessed that she had felt the same way about him all along. Since they could never marry, she told him she would rather go to the pure land than marry a man who was not her brother, and she took his place as the sacrifice. Her motivation is a combination of love for Genkei and despair over the impossibility of a life together; she chose death as the only way to escape her feelings and her circumstances.

In the story, Oyo never appears in the present except through memory and the physical evidence of the recovered hollow boat. Her choice to sacrifice herself in Genkei's place is the pivotal event that defines her brother's life and drives the events of the arc. After the boat is dragged up from the sea fifty years later, it is found to be empty, revealing that Oyo had in reality given herself to the sea, as she too could no longer continue living with her feelings for her brother. This revelation is essential to understanding the truth behind the mononoke, because Genkei's guilt, his hidden relief that his sister died in his place, and his inability to truly love her are what created the deadly supernatural sea. Oyo therefore functions as the absent emotional center of the arc; her life and death explain the origins of the haunting that the Medicine Seller must resolve.

Her key relationship is with Genkei, and nearly everything about her character is defined through that bond. She has no notable supernatural or combat abilities, and she is not a living participant in the main events of the story. Instead, her significance lies in her actions and their consequences. Her story is revealed retrospectively through Genkei's account and the discovery of the empty boat, and through that gradual revelation her decision to take her brother's place becomes the key that unlocks the arc's central mystery.