| Ohio | 42,000 | Wheeling | West Virginia | Wheeling-Clarksburg-Parkersburg, WV | View → |
| Okaloosa | 223,800 | Fort Walton Beach | Florida | Mobile, AL-Panama City, FL | View → |
| Okanogan | 44,000 | Omak | Washington | Spokane, WA | View → |
| Okeechobee | 44,000 | Okeechobee | Florida | Miami-West Palm Beach, FL | View → |
| Okfuskee | 12,000 | Okemah | Oklahoma | Tulsa, OK-Fort Smith, AR | View → |
| Oklahoma | 806,245 | Oklahoma City | Oklahoma | Oklahoma City, OK | View → |
| Okmulgee | 39,250 | Okmulgee | Oklahoma | Tulsa, OK-Fort Smith, AR | View → |
| Oktibbeha | 49,000 | Starkville | Mississippi | Tupelo-Meridian-Greenwood, MS | View → |
| Oldham | 2,000 | Vega | Texas | Amarillo, TX | View → |
| Oldham | 70,000 | La Grange | Kentucky | Louisville, KY-Evansville, IN | View → |
| Oliver | 2,000 | Center | North Dakota | Bismarck, ND | View → |
| Olmsted | 166,000 | Rochester | Minnesota | Madison-La Crosse, WI | View → |
| Oneida | 228,600 | Utica | New York | Buffalo-Rochester-Syracuse, NY | View → |
| Oneida | 36,000 | Rhinelander | Wisconsin | Green Bay, WI-Duluth, MN | View → |
| Oneida | 4,600 | Malad City | Idaho | Salt Lake City, UT | View → |
| Onondaga | 460,000 | Syracuse | New York | Buffalo-Rochester-Syracuse, NY | View → |
| Onslow | 210,000 | Jacksonville | North Carolina | Raleigh-Greenville-Wilmington, NC | View → |
| Ontario | 110,000 | Canandaigua | New York | Buffalo-Rochester-Syracuse, NY | View → |
| Ontonagon | 5,800 | Ontonagon | Michigan | Marquette, MI | View → |
| Orange | 29,000 | Randolph | Vermont | Burlington, VT | View → |
| Orange | 86,000 | Orange | Texas | Houston, TX | View → |
| Orange | 3,216,700 | Santa Ana | California | Los Angeles-San Diego, CA | View → |
| Orange | 19,000 | Paoli | Indiana | Louisville, KY-Evansville, IN | View → |
| Orange | 400,000 | Middletown | New York | New York, NY | View → |
| Orange | 1,512,000 | Orlando | Florida | Orlando-Gainesville, FL | View → |
| Orange | 150,000 | Chapel Hill | North Carolina | Raleigh-Greenville-Wilmington, NC | View → |
| Orange | 40,000 | Orange | Virginia | Richmond, VA | View → |
| Orangeburg | 86,000 | Orangeburg | South Carolina | Columbia, SC | View → |
| Oregon | 10,500 | Alton | Missouri | Springfield, MO | View → |
| Orleans | 40,000 | Albion | New York | Buffalo-Rochester-Syracuse, NY | View → |
| Orleans | 27,000 | Newport | Vermont | Burlington, VT | View → |
| Orleans | 390,000 | New Orleans | Louisiana | New Orleans-Baton Rouge, LA | View → |
| Osage | 15,500 | Osage City | Kansas | Kansas City, MO | View → |
| Osage | 14,000 | Linn | Missouri | St. Louis, MO | View → |
| Osage | 47,000 | Pawhuska | Oklahoma | Tulsa, OK-Fort Smith, AR | View → |
| Osborne | 3,500 | Osborne | Kansas | Wichita, KS | View → |
| Osceola | 438,000 | Kissimmee | Florida | Orlando-Gainesville, FL | View → |
| Osceola | 6,700 | Sibley | Iowa | Sioux Falls-Rapid City, SD | View → |
| Osceola | 23,000 | Reed City | Michigan | Traverse City, MI | View → |
| Oscoda | 8,500 | Mio | Michigan | Traverse City, MI | View → |
| Oswego | 117,600 | Oswego | New York | Buffalo-Rochester-Syracuse, NY | View → |
| Otero | 66,000 | Alamogordo | New Mexico | Albuquerque, NM | View → |
| Otero | 18,000 | La Junta | Colorado | Denver-Colorado Springs, CO | View → |
| Otoe | 16,000 | Nebraska City | Nebraska | Omaha, NE-Sioux City, IA | View → |
| Otsego | 58,000 | Oneonta | New York | Albany-Utica-Elmira, NY | View → |
| Otsego | 25,000 | Gaylord | Michigan | Traverse City, MI | View → |
| Ottawa | 41,000 | Port Clinton | Ohio | Detroit, MI-Toledo, OH | View → |
| Ottawa | 310,000 | Holland | Michigan | Grand Rapids, MI | View → |
| Ottawa | 31,500 | Miami | Oklahoma | Tulsa, OK-Fort Smith, AR | View → |
| Ottawa | 5,700 | Minneapolis | Kansas | Wichita, KS | View → |
