| State Name | County Name | City |
|---|---|---|
| Nebraska | Holt | O'Neill |
| Alabama | Blount | Oneonta |
| New York | Otsego | Oneonta |
| New York | Onondaga | Onondaga |
| California | San Bernardino | Ontario |
| New York | Wayne | Ontario |
| Ohio | Richland | Ontario |
| Oregon | Malheur | Ontario |
| Wisconsin | Sheboygan | Oostburg |
| Florida | Miami-Dade | Opa-locka |
| Alabama | Lee | Opelika |
| Louisiana | St. Landry | Opelousas |
| Alabama | Covington | Opp |
| Arizona | Pinal | Oracle |
| New Jersey | Bergen | Oradell |
| California | Orange | Orange |
| Connecticut | New Haven | Orange |
| Massachusetts | Franklin | Orange |
| Ohio | Cuyahoga | Orange |
| Texas | Orange | Orange |
| Virginia | Orange | Orange |
| Alabama | Baldwin | Orange Beach |
| Iowa | Sioux | Orange City |
| California | Fresno | Orange Cove |
| New York | Orange | Orange Lake |
| Florida | Clay | Orange Park |
| New York | Rockland | Orangeburg |
| South Carolina | Orangeburg | Orangeburg |
| New York | Rockland | Orangetown |
| Florida | Collier | Orangetree |
| California | Sacramento | Orangevale |
| Colorado | Delta | Orchard City |
| Montana | Missoula | Orchard Homes |
| Colorado | Mesa | Orchard Mesa |
| New York | Erie | Orchard Park |
| Washington | Clark | Orchards |
| Hawaii | Hawaii | Orchidlands Estates |
| California | Santa Barbara | Orcutt |
| Illinois | Ogle | Oregon |
| Ohio | Lucas | Oregon |
| Wisconsin | Dane | Oregon |
| Oregon | Clackamas | Oregon City |
| Pennsylvania | Montgomery | Oreland |
| Utah | Utah | Orem |
| California | Contra Costa | Orinda |
| California | Glenn | Orland |
| Illinois | Cook | Orland Hills |
| Illinois | Cook | Orland Park |
| Florida | Orange | Orlando |
| Massachusetts | Barnstable | Orleans |
| Florida | Orange | Orlovista |
| Florida | Volusia | Ormond Beach |
| Florida | Volusia | Ormond-by-the-Sea |
| Arizona | Pima | Oro Valley |
| Idaho | Clearwater | Orofino |
| Maine | Penobscot | Orono |
| Minnesota | Hennepin | Orono |
| California | Tulare | Orosi |
| California | Butte | Oroville |
| California | Butte | Oroville East |
| Maine | Penobscot | Orrington |
| Ohio | Wayne | Orrville |
| Washington | Pierce | Orting |
| Pennsylvania | Schuylkill | Orwigsburg |
| Iowa | Mitchell | Osage |
| Missouri | Camden | Osage Beach |
| Kansas | Osage | Osage City |
| Kansas | Miami | Osawatomie |
| Arkansas | Mississippi | Osceola |
| Iowa | Clarke | Osceola |
| Wisconsin | Polk | Osceola |
| Wisconsin | Winnebago | Oshkosh |
| Iowa | Mahaska | Oskaloosa |
| Florida | Sarasota | Osprey |
| Indiana | Wells | Ossian |
| New York | Westchester | Ossining |
| New Hampshire | Carroll | Ossipee |
| New York | St. Lawrence | Oswegatchie |
| Illinois | Kendall | Oswego |
| New York | Oswego | Oswego |
| New York | Otsego | Otego |
| Washington | Adams | Othello |
| Washington | Spokane | Otis Orchards-East Farms |
| New York | Orange | Otisville |
| Michigan | Allegan | Otsego |
| Minnesota | Wright | Otsego |
| New York | Otsego | Otsego |
| Illinois | LaSalle | Ottawa |
| Kansas | Franklin | Ottawa |
| Ohio | Putnam | Ottawa |
| Ohio | Lucas | Ottawa Hills |
| Iowa | Wapello | Ottumwa |
| Missouri | St. Louis | Overland |
| Kansas | Johnson | Overland Park |
| Maryland | Baltimore | Overlea |
| Florida | Seminole | Oviedo |
| Texas | Ellis | Ovilla |
| New York | Cayuga | Owasco |
| Oklahoma | Tulsa | Owasso |
| Florida | Volusia | Orange City |
