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- Brandy O'Bannon is a reporter in a series set in Florida by Ann Turner Cook, commenced 2001.
- Kate O'Donnell is a photographer working in 1960s London in a new series of books by Patricia Hall first appearing in 2011.
- Maurenn O’Donnell is an engaging survivor in three books set in Glasgow by Denise Mina.
- Shay O'Hanlon is a coffee shop owner drawn into various capers in a series set in Minnesota by Jessie Chandler, commencing in 2011.
- Juliet O'Hara is a police detective in Santa Barbara, California played by Maggie Lawson on the 2006-present USA series Psych.
- Ariadne Oliver is a detective novelist who attempts to assist and guide Hercule Poirot in some of Agatha Christie’s novels, but who usually gets things thoroughly wrong. A lot of her books are parodies of Agatha's own work.
- Kit O’Malley is a private investigator and former policewoman in Melbourne, Australia, debuted 1998 in a book series by Lindy Cameron.
- Peggy O’Neil is a campus cop in a series by M D Lake debuted 1989.
- Christie Opara is a police detective in New York City in a trilogy of books by American crime novelist and former police detective Dorothy Uhnak (debuted 1968; last book 1970).
- Rita Ortiz is a New York City detective played by Jacqueline Obradors on the ABC series NYPD Blue from 2001 to 2005.
Read more about this topic: List Of Female Detective Characters