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- Onondaga Castle, New York
- Open Range
- Opening the West
- Operation Barbarossa
- Operation Barbarossa
- Operation PUSH (People United to Serve Humanity)
- Ophüls, Marcel (b. 1927)
- Opinion Polls and Mass-Observation
- Oppenheimer, Ernest
- Opportunity: Journal of Negro Life
- Oquaga (Onoquaga), New York
- Orangeburg, South Carolina
- Orangetown, New York
- Ordin-Nashchokin, Afanasy Lavrentievich
- Ordzhonikidze, Grigory Konstantinovich
- Oregon Country Cessation
- Orff, Carl (1895–1982)
- Organisation for European Economic Cooperation (OEEC)
- Organización Regional Interamericana de Trabajadores