- Barry O (akas: The Zodiac, Barrymore Barlow, Barry "O" Bazza, The Highroller) – Randal Barry Orton
- Jimmy O – Jean Jimmy Alexandre
- Karen O – Karen Lee Orzolek
- Jack Oakie – Lewis Delaney Offield
- Annie Oakley – Phoebe Ann Moses
- Merle Oberon – Estelle Merle O'Brien Thompson
- Hugh O'Brian – Hugh Charles Krampe
- Jim O'Brien (radio) – James Oldham
- Margaret O'Brien – Angela Maxine O'Brien
- Richard O'Callaghan – Richard Rooke
- Billy Ocean – Leslie Sebastian Charles
- Frank Ocean – Christopher Breaux, later legally changed to Christopher Francis Ocean
- Una O'Connor – Agnes Teresa McGlade
- Anita O'Day – Anita Belle Colton
- Dawn O'Day – Dawn Evelyeen Paris (O'Day, a child actor, would be known as an adult actor under the name Anne Shirley)
- Molly O'Day (actress) – Suzanne Dobson Noonan
- Molly O'Day (singer) – Lois LaVerne Williamson
- Cathy O'Donnell – Ann Steely
- Lani O'Grady – Lanita Rose Agrati
- Nivek Ogre – Kevin Graham Ogilvie
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- Jenny O'Hara – Patricia Joanne O'Hara
- Maureen O'Hara – Maureen Fitzsimons
- Dead Ohlin (a.k.a. "Dead") – Per Yngve Ohlin
- Dennis O'Keefe – Edward Vance Flanagan
- Okito – Tobias Leendert Bamberg
- Ol' Dirty Bastard – Russell Tyrone Jones
- Warner Oland – Johan Verner Ölund
- Edna May Oliver – Edna May Nutter
- King Oliver – Joseph Nathan Oliver
- Susan Oliver – Charlotte Gercke
- Sy Oliver – Melvin Oliver
- Nance O'Neil (a.k.a. Nancy O'Neil) — Gertrude Lamson
- Paddie O'Neil – Adalena Lillian Nail
- Sally O'Neil – Virginia Louise Concepta Noonan
- Maire O'Neill – Mary Allgood
- Tara Lynne O'Neill – Lynne James
- Anny Ondra – Anna Sophie Ondráková
- Max Ophüls – Maximillian Oppenheimer
- Opie – Gregg Hughes
- Terry O'Quinn – Terrance Quinn
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- Tony Orlando – Michael Anthony Orlando Cassavitis
- Kid Ory – Edward Ory
- Osang – Jennifer Adriano-Molina (a.k.a. Rosanna Roces)
- Super Dave Osborne – Robert Kel Einstein
- Ozzy Osbourne – John Michael Osbourne
- Sharon Osbourne – Sharon Rachel Levy
- Henry Oscar – Henry Wale
- Lee Oskar – Oskar Levetin Hansen
- Gilbert O'Sullivan – Raymond Edward O'Sullivan
- Annette O'Toole – Annette Toole
- Johnny Otis – John Alexander Veliotes
- Shuggie Otis – Johnny Alexander Veliotes
- Natalino Otto – Natale Codognotto
- Bill Owen – William John Owen Rowbotham
- Glyn Owen – Griffith Owen
- Seena Owen – Signe Auen
- Elizabeth Owens (a.k.a. Kathe Feist) – Irma Schneider
- Buck Owens – Alvis Edgar Owens, Jr.
- Gary Owens – Gary Altman
- Frank Oz – Richard Frank Oznowicz
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