Inspector O: state police officer in North Korea, by James Church

Blackford “Blackie” Oakes: CIA agent recruited from Yale, in Washington DC, by William Buckley Jr.

Boysie Oakes: squeamish government assassin in England, by John Gardner

Iris Oakley: zookeeper and amateur sleuth at Finley Memorial Zoo, near Vancouver, Washington, by Ann Littlewood

Margot O’Banion: film editor, and Max Skull, a movie director, based in Los Angeles, California. by Kit Sloane

Felicity O’Brian: an Irish jewel thief, and Morgan Stark, a black mercenary soldier, by Austin S. Camacho

Emerald O’Brien: tea shop owner and psychic, in the Chintz’n China Mysteries, by Yasmine Galenorn

Kali O’Brien: attorney in Gold Country, California, by Jonnie Jacobs

Morgan O’Brien: Canadian government investigator specializing in science fraud, based in Ottawa, Ontario, by Alex Brett

Sean O’Brien: recently widowed former Miami homicide detective, now living with his dachshund Max on the St. John’s River, in Florida, by Tom Lowe

Mairead O’Clare: lawyer, and Sheldon Gold, her mentor, in Boston, Massachusetts, by Terry Devane (Jeremiah Healy)

Cork O’Connor: Native American ex-sheriff in Aurora, Minnesota, by William Kent Krueger 

Dallas O’Connor: freelance fashion stylist in Texas, by Chloe Green

Kieran O’Connor: freelance journalist in Hollywood, California, by Kevin Allman

Rachel O’Connor: landscaper in Blossom, Oregon, in the Gardening mysteries, by Mary Freeman

Rita Gardella O’Dea: mafia princess in Massachusetts, by Andrew Coburn

Mike Oddie: Police Superintendent in Leeds, England, by Robert Barnard

Kendall O’Dell: reporter at a small-town newspaper in fictional Castle Valley, Arizona, by Sylvia Nobel

Maggie O’Dell: FBI Criminal Profiler, by Alex Kava

Dae O’Donnell: the mayor of the small town of Duck, North Carolina, who runs a collectible shop and uses her psychic abilities to find lost things, in the Missing Pieces mysteries by Joyce and Jim Lavene

Kate O’Donnell: aspiring photographer in 1960s London, England, by Patricia Hall

Maureen O’Donnell: an unlikely crime solver who is disheveled, mumbling to herself, and drunk by noon, in Glasgow, Scotland, in the Garnethill Trilogy by Denise Mina

Smoky O’Donnell: small-town narcotics officer, in Farmer's Mound, Texas, by Martha G. Webb (Anne Wingate)

Danny O’Flaherty: American teacher researching his family’s roots in Ireland and in New York City, by Jonathan Harrington

Lee Ofsted: a struggling woman golfer and Graham Sheldon, her cop boyfriend, by Aaron Elkins and Charlotte Elkins

Sam O’Gara, Joe Crow, Axel Speeter, and Tommy Fabian, a group of small-town gamblers in Minnesota, by Peter Hautman

Katrina (K.O.) Ogden: Honolulu police officer in various locations in Hawaii, by Victoria Heckman

Ned “Dog” Oglivie: traveling the United States in a 1984 RV, trout fishing until his money runs out, in the Fly Fishing mysteries by John Galligan

Deirdre O’Hara: lawyer specializing in marine insurance cases, in Montreal, Quebec, Canada, by Maurice Gagnon

Kate O’Hare: FBI agent and former Navy Seal, and Nick Fox, an international thief and con man, in the Fox & O’Hare mysteries by Janet Evanovich and Lee Goldberg

Michael Ohayon: chief inspector in Jerusalem, Israel, by Batya Gur

Bridget “Bridg” O’Hern: troubleshooter and consultant with nonprofit organizations, by Patricia Harrington

Maggie Olenski: young high school math teacher in Baltimore, Maryland, by Mary Ellen Hughes

Billy Oliphant: ex-cop, security consultant and occasional private investigator, in England, by Alan Dunn

Gideon Oliver: anthropology professor in Port Angeles, Washington, by Aaron Elkins

Elena Oliverez: Director of Museum of Mexican Arts in Santa Barbara, California, by Marcia Muller

Shay O’Hanlon: co-owner of The Rabbit Hole, a quirky-cool coffee shop, and amateur sleuth, along with JT Bordeaux, her cop girlfriend, in Minneapolis, Minnesota, by Jessie Chandler

Annie O’Hara: owner of an antique shop, and Claudius, “the Hound from Hell,” by Ann Campbell

Darlene O’Hara, a homicide detective in the 7th precinct, and her partner, Serge “K.” Krekorian, in New York City, by Peter de Jonge

Jacqueline “Jake” O’Hara: ghost writer in Manhattan, New York City, by Noreen Wald

Andi Oliver: young amnesiac and drifter, by Martha Grimes

Superintendent O’Malley, of the Irish Garda, by Michael Kenyon

John Aloysius O’Malley: a Jesuit missionary, and Vicky Holden, an Arapaho attorney, on the Wind River Reservation, Wyoming, by Margaret Coel

Kit O’Malley: former police officer, now a private investigator in Melbourne, Australia, by Lindy Cameron

Jay Omega: college professor and science-fiction author, by Sharyn McCrumb

Freddie O’Neal: private investigator in Reno, Nevada, by Catherine Dain

Allison O’Neil: lesbian Bed & Breakfast owner and refugee from California, in fictional Lavner Bay, Oregon, by Lauren Wright Douglas

Connor O’Neill: chief inspector, and Fran Wilson, detective sergeant, in Fowchester, England, by Christine Green

Jim O’Neill: police officer in Connecticut, by Doris Miles Disney

Peggy O’Neill: university campus cop in Minnesota, by M.D. Lake

Christie Opara: police officer in New York City, by Dorothy Uhnak

Monette O’Reilley: a towering lesbian and star player of the Leaping Lesbians soccer team and graphic artist in New York, by David Stukas

Jocelyn O’Roarke: actress in New York City, by Jane Dentinger

Grove O’Rourke: stockbroker at the investment firm of Sachs, Kidder, and Carnegie, in New York City, by Norb Vonnegut

Terry Orr: newly-licensed private investigator, and his daughter Bella, in Manhattan, New York, by Jim Fusilli

Johnny Ortiz: half-Apache, half-Chicano, police lieutenant, around Santa Fe, New Mexico, by Richard Martin Stern

Thea Osborne: house-sitter in the Cotswolds, England, by Rebecca Tope

Michael Osbourne: CIA agent in Washington DC, by Daniel Silva

Kiernan O’Shaughnessy: former San Francisco medical examiner turned private investigator, in La Jolla, California, by Susan Dunlap

Victory (Torie) O’Shea: genealogist in New Kassel, Missouri, by Rett MacPherson

Lee Henry Oswald: Gulf War vet, loner, and private investigator, in Dallas, Texas, by Harry Hunsicker

Tetsuo Otani: Superintendent of Police in Kobe, Japan, by James Melville

Lucky O’Toole: statuesque beauty in charge of customer relations for the mega-casino Babylon Hotel, on the Strip in Las Vegas, Nevada, by Deborah Coonts

Rev. Francis Oughterard: the vicar of Molehill and a flustered murderer and blackmail victim, along with Bouncer the dog and Maurice the cat, in 1950s Surrey, England, by Suzette A. Hill

Debbie Sue Overstreet: doing big hair at the Styling Station, and her best pal Edwina Perkins-Martin, later private investigators, in fictional Salt Lick, Texas, by Dixie Cash

Bill Owen, the Old Man in the Corner, an armchair detective who sits in a restaurant playing with string, and relates the solution of crimes to Polly Burton, a young journalist, in London, England, by Baroness Orczy

Bobby Owen, an Oxford graduate, starting as a police constable, later a detective sergeant then Commander of Scotland Yard, mostly in London, England, by E.R. Punshon

Garth Owen (Mamur Zapt): the British head of Cairo’s Political CID in Cairo, Egypt, by Michael Pearce

Laura Owen: Commissioner of Public Safety in Mississippi, by Louisa Dixon

Samantha Owens: medical examiner from Tennessee, by J.T. Ellison

Jack Oxby: Scotland Yard art forgery investigator in London, England, by Thomas Swan

Dr. Saylor Oz: a sex therapist, and her roommate, Benita Morales, a financial analyst and boxer, in Brooklyn and Manhattan, New York City, by Allyson Roy