- Oakland, Alameda County, California1 objects
- Obulco (ancient place), Hispania Ulterior2 objects
- Ocumicho, Michoacan, Mexico1 objects
- Odessus, ancient Thrace2 objects
- Ogallala, Keith County, Nebraska1 objects
- Oilaunes (ancient place)1 objects
- Oreus (ancient place)1 objects
- Orinda, Contra Costa County, California2 objects
- Orippo (ancient place), Hispania Ulterior1 objects
- Orvieto, Umbria, Central Italy1 objects
- Osaka Prefecture, Kansai region2 objects
- Osca (ancient place)1 objects
- Oschersleben, Saxony-Anhalt, Germany8 objects
- Ostseebad Arendsee, Mecklenburg-Vorpommern, Germany3 objects
- Ostseebad Graal-Müritz, Mecklenburg-Vorpommern, Germany3 objects
- Ottensheim, Upper Austria4 objects