Ontario 418
Outer space 318
Ohio 299
Oceania 148
Oregon 134
Oklahoma 76
Oakland 45
OECD countries 36
Ohio River Valley 36
Oman 31
Oxfordshire 25
Oxfordshire (England) 25
Oakland (Calif.) 21
Oxford 20
Oaxaca (State) 17
Orient 17
Oaxaca (Mexico : State) 12
Ozark Mountains 12
Oak Park (Ill.) 11
Ohio River 11
Oak Park 10
Okinawa Island 10
Oahu 9
Ontario, Northern 9
Odesa 7
Ottawa (Ont.) 7
Oxford (England) 7
Odesa (Ukraine) 6
Okavango River Delta 6
Okavango River Delta (Botswana) 6
Oregon National Historic Trail 6
Oaxaca Valley 5
Okinawa Island (Japan) 5
Okinawa-ken (Japan) 5
Oklahoma City 5
Omaha 5
Omaha (Neb.) 5
Ontario, Lake (N.Y. and Ont.) 5
Ontario, Southern 5
Oslo 5
Oaxaca Valley (Mexico) 4
Odisha 4
Odisha (India) 4
Ohio and Erie Canal (Ohio) 4
Omo River Valley (Ethiopia and Kenya) 4
Orinoco River (Venezuela and Colombia) 4
Orkney (Scotland) 4
Ottawa 4
Oberlin 3
Okavango River 3
Okinawa-ken 3
Orange County 3
Orkney 3
Orlando 3
Oss 3
Ostia (Extinct city) 3
Outer Banks (N.C.) 3
Oxford (Miss.) 3
Oxnard 3
Oświęcim 3
Oahu (Hawaii) 2
Oak Ridge 2
Oak Ridge (Tenn.) 2
Oberlin (Ohio) 2
Oceana County 2
Oceana County (Mich.) 2
Ocmulgee Mounds National Historical Park 2
Ocmulgee Mounds National Historical Park (Ga.) 2
Oder-Neisse Line (Germany and Poland) 2
Ogallala Aquifer 2
Ohio River Region 2
Ohio, Southeastern 2
Oklahoma City National Memorial (Okla.) 2
Olduvai Gorge 2
Olympic National Park (Wash.) 2
Olympic Peninsula (Wash.) 2
Oran 2
Oran (Algeria) 2
Orange County (Calif.) 2
Orangeburg 2
Orangeburg (S.C.) 2
Oregon Territory 2
Orinoco River Valley (Venezuela and Colombia) 2
Osaka (Japan) 2
Oshogbo 2
Oss (Netherlands) 2
Otavalo 2
Ottawa River (Québec and Ont.) 2
Ottawa River Valley (Québec and Ont.) 2
Otway Range (Vic.) 2
Ouagadougou 2
Outer Banks 2
Overland Trails 2
Owens Valley 2
Oyotunji African Village (S.C.) 2
Oak Lawn 1
Oak Lawn (Ill.) 1
Oakfuskee 1
Oakfuskee (Ala.) 1
Oaxaca de Juárez 1
