- O'Brien
- O'Fallon
- Oak Brook
- Oak Creek
- Oak Forest
- Oak Grove
- Oak Harbor
- Oak Hill
- Oak Lawn
- Oak Park
- Oak Park Heights
- Oak Point
- Oak Ridge
- Oak Ridge North
- Oakboro
- Oakbrook Terrace
- Oakdale
- Oakfield
- Oakford
- Oakhurst
- Oakland
- Oakland City
- Oakland Park
- Oakleigh South
- Oakley
- Oaklyn
- Oakmont
- Oakridge
- Oakton
- Oaktown
- Oakville
- Oakwood
- Oakwood Village
- Oberlin
- Obetz
- Oblong
- Ocala
- Ocean Springs
- Ocean View
- Oceanport
- Oceanside
- Ocoee
- Oconomowoc
- Oconto
- Oconto Falls
- Odenton
- Odenville
- Odessa
- Odon
- Oelwein
- Ogden
- Ogden Dunes
- Ogdensburg
- Ohio City
- Ojai
- Okarche
- Okawville
- Okeana
- Okeechobee
- Okemah
- Oklahoma City
- Olathe
- Old Bridge Township
- Old Lyme
- Old Mission
- Old Saybrook
- Old Tappan
- Old Town
- Old Westbury
- Olde West Chester
- Oldenburg
- Olds
- Oldsmar
- Oldtown
- Olean
- Oley
- Olive Branch
- Oliver Springs
- Olivet
- Olivette
- Olmsted Falls
- Olney
- Olten
- Olympia
- Olympia Fields
- Olyphant
- Omaha
- Ona
- Onalaska
- Onamia
- Oneida
- Oneonta
- Ono
- Onsted
- Ontario
- Onward
- Oolitic
- Ooltewah
- Opa-locka
- Opelika
- Opp
- Ora
- Oradell
- Oran
- Orange
- Orange Beach
- Orange City
- Orange Park
- Orangeburg
- Orangevale
- Orangeville
- Orbisonia
- Orchard Grass Hills
- Orchard Lake Village
- Orchard Park
- Oregon
- Oregon City
- Oregonia
- Orem
- Orestes
- Oriental
- Orinda
- Orion charter Township
- Oriskany
- Orland
- Orland Hills
- Orland Park
- Orlando
- Orleans
- Ormond Beach
- Oro Valley
- Orrville
- Orwell
- Osage Beach
- Osage City
- Osakis
- Osborne
- Osceola
- Oscoda Township
- Osgood
- Oshawa
- Oshkosh
- Oskaloosa
- Osprey
- Osseo
- Ossian
- Ossining
- Ostrander
- Oswego
- Ōta-ku
- Otisco
- Otisville
- Otsego
- Ottawa
- Ottawa Hills
- Ottawa Lake
- Otterbein
- Otto
- Ottumwa
- Otwell
- Oulu
- Overland
- Overland Park
- Ovid
- Oviedo
- Ovilla
- Owasso
- Owatonna
- Owen Sound
- Owens Cross Roads
- Owensboro
- Owensburg
- Owensville
- Owenton
- Owings
- Owings Mills
- Owingsville
- Owosso
- Oxford
- Oxnard
- Oxon Hill
- Oyster Bay
- Ozark