- Oberlander, Helmut: 1
- occupation zone: 1, 2
- Oggins, Isai: 1, 2
- OGPU (United State Political Directorate): 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18
- Ohlendorf, Otto: 1, 2
- OKR Smersh: 1, 2, 3
- Okulicki, Leopold: 1
- Olsky, Yan: 1, 2, 3
- OO (Special Department): 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51
- Operation Ariitsy: 1
- Operation Barbarossa: 1, 2, 3
- Operation Flamingo: 1
- Operation Monastyr’: 1
- Operation Zeppelin: 1, 2, 3, 4
- Order No. 1: 2, 3, 4
- Order No. 1: 2
- Orpo (German Order Police): 1, 2, 3
- Osetrov, Nikolai: 1, 2, 3, 4, 5
- OSO (Special Board): 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26
- osobist (OO officer): 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14
- OSS (Office of Strategic Service): 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
- Ostashkov: 1, 2
- OVD (Department for Especially Important Cases): 1, 2
-
- Oxley, Walter H.: 1