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| Oakey (AU) | Olpoi (VU) | O-shima (JP) |
| Oak Harbor (US) | Omaha (US) | Osijek (HR) |
| Oakland (US) | Omsk (RU) | Oskarshamn (SE) |
| Oamaru (NZ) | Ondangwa (NA) | Oskemen Qalasy (KZ) |
| Oaxaca (MX) | Ontario (US) | Oslo (NO) |
| Oban (GB) | Oradea (RO) | Osorno (CL) |
| Obihiro (JP) | Oral (KZ) | Ostend (BE) |
| Obo (PG) | Oran (DZ) | Ostrava (CZ) |
| Odate (JP) | Orange (AU) | Ottawa (CA) |
| Odessa (UA) | Oranjemund (NA) | Ouagadougou (BF) |
| Ogdensburg (US) | Orenburg (RU) | Ouargla (DZ) |
| Ohrid (MK) | Orkney Islands (GB) | Ouarzazate (MA) |
| Oita (JP) | Orland (NO) | Oujda (MA) |
| Okayama (JP) | Orlando (US) | Oulu (FI) |
| Okinawa (JP) | Ormoc City (PH) | Ouzinkie (US) |
| Oklahoma City (US) | Ornskoldsvik (SE) | Owe (NG) |
| Olbia (IT) | Orsk (RU) | Owensboro (US) |
| Old Crow (CA) | Osaka (JP) | Oxford House (CA) |
| Old Harbor (US) | Osh (KG) | Ozamis (PH) |