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- Ogallala Aquifer1
- OhioChampaign County2
- OhioClark County2
- OhioDarke County1
- OhioDayton1
- OhioGreene County1
- OhioLucas County1
- OhioMontgomery County1
- OhioPreble County1
- OhioSandusky County1
- OhioWood County1
- OhioWright-Patterson Air Force Base Region1
- Oklahoma1
- Ontario, Lake (N.Y. and Ont.)1
- Ontario, Lake, Region (N.Y. and Ont.)1
- Oregon2
- OregonHarney County1
- Oro, CanĚada del (Ariz.)1
- Ouachita National Forest (Ark. and Okla.)1