Appears in the story "The Case of Yoshiko and Haruko". Oohara is a girl in a grade above Haruko and Yoshiko, who also went on a hunt for a mermaid but was caught by them and turned into one. She appears to Yoshiko as an "O" mermaid — same as the initial of Ogura, Yoshiko's crush. Oohara appears briefly in the second story as well, where she gets the notebook and it is revealed that she also likes Kimura.