Born on an American military base in Tokyo to Japanese and Chinese parents, young O-Ren Ishii watched her parents get brutally murdered by the Yakuza boss; Boss Matsumoto. She vowed revenge and a couple of years later succeeded in killing Matsumoto.
By the age of twenty, O-Ren was one of the world's deadliest assassins and became a protege of Bill, joining his Deadly Viper Assassination Squad under the codename: Cottonmouth. O-Ren did not know it at the time, but she sowed her fate when along with Bill and the rest of the Deadly Vipers she wiped out an entire wedding party in El Paso, Texas but failed to kill Beatrix Kiddo, a former Deadly Viper who was called Black Mamba.
A year later, Bill backed O-Ren financially and philisophically in her bid to take control of the Yakuza clans of Tokyo. The take-over succeeded, and O-Ren was both a ruthless but fair leader, under her the only taboo topic was to speak ill of her multi-cultural heritage. Boss Tanaka learned that lesson the hard way, as he lost his head.
Three years later, Beatrix awoke from the coma that Bill's last bullet put her in, and now called The Bride sought revenge against all the Deadly Vipers. Her first target was O-Ren, and she was easily able to track her down. The Bride fought her way past O-Ren's personal army, the Crazy 88, her body guard Gogo Yubari, before making her way to O-Ren. Both armed with swords forged by the smith Hattori Hanzo, the two fought a duel to the death, and in the end it was O-Ren who fell.
- O-Ren Ishii was portrayed by actor Lucy Liu.
- Kill Bill Vol 1: