O
| Oakey QL (AU) | Olympic Dam SA (AU) | Osijek (HR) |
| Oak Harbor WA (US) | Omaha NE (US) | Oskarshamn (SE) |
| Oakland CA (US) | Omsk (RU) | Oskemen (KZ) |
| Oamaru (NZ) | Ondangwa (NA) | Oslo (NO) |
| Oaxaca (MX) | Ontario CA (US) | Osorno (CL) |
| Oban (GB) | Oradea (RO) | Ostend (BE) |
| Obihiro (JP) | Oral (KZ) | Ostersund (SE) |
| Obo (PG) | Oran (DZ) | Ostrava (CZ) |
| Odate (JP) | Orange NS (AU) | Ottawa ON (CA) |
| Odessa (UA) | Oranjemund (NA) | Ouagadougou (BF) |
| Ohrid (MK) | Orebro (SE) | Ouargla (DZ) |
| Oita (JP) | Orenburg (RU) | Ouarzazate (MA) |
| Okayama (JP) | Orkney Island (GB) | Oudomxay (LA) |
| Okhotsk (RU) | Orland (NO) | Oujda (MA) |
| Oki Island (JP) | Orlando FL (US) | Oulu (FI) |
| Okinawa (JP) | Ornskoldsvik (SE) | Ouvea (NC) |
| Oklahoma City OK (US) | Orsk (RU) | Ouzinkie AK (US) |
| Olbia (IT) | Orsta-Volda (NO) | Owerri (NG) |
| Old Crow YT (CA) | Osaka (JP) | Oxford House MB (CA) |
| Old Harbor AK (US) | Osh (KG) | Ozamis City (PH) |
| Olpoi (VU) | Oshima (JP) |