User:Juliancolton/List of counties/P
- O'Brien County, Iowa
- Oakland County, Michigan
- Obion County, Tennessee
- Ocean County, New Jersey
- Oceana County, Michigan
- Ochiltree County, Texas
- Oconee County, Georgia
- Oconee County, South Carolina
- Oconto County, Wisconsin
- Ogemaw County, Michigan
- Ogle County, Illinois
- Oglethorpe County, Georgia
- Ohio County, Indiana
- Ohio County, Kentucky
- Ohio County, West Virginia
- Okaloosa County, Florida
- Okanogan County, Washington
- Okeechobee County, Florida
- Okfuskee County, Oklahoma
- Oklahoma County, Oklahoma
- Okmulgee County, Oklahoma
- Oktibbeha County, Mississippi
- Oldham County, Kentucky
- Oldham County, Texas
- Oliver County, North Dakota
- Olmsted County, Minnesota
- Oneida County, Idaho
- Oneida County, New York
- Oneida County, Wisconsin
- Onondaga County, New York
- Onslow County, North Carolina
- Ontario County, New York
- Ontonagon County, Michigan
- Orange County, California
- Orange County, Florida
- Orange County, Indiana
- Orange County, New York
- Orange County, North Carolina
- Orange County, Texas
- Orange County, Vermont
- Orange County, Virginia
- Orangeburg County, South Carolina
- Oregon County, Missouri
- Orleans County, New York
- Orleans County, Vermont
- Orleans Parish, Louisiana
- Osage County, Kansas
- Osage County, Missouri
- Osage County, Oklahoma
- Osborne County, Kansas
- Osceola County, Florida
- Osceola County, Iowa
- Osceola County, Michigan
- Oscoda County, Michigan
- Oswego County, New York
- Otero County, Colorado
- Otero County, New Mexico
- Otoe County, Nebraska
- Otsego County, Michigan
- Otsego County, New York
- Ottawa County, Kansas
- Ottawa County, Michigan
- Ottawa County, Ohio
- Ottawa County, Oklahoma
- Otter Tail County, Minnesota
- Ouachita County, Arkansas
- Ouachita Parish, Louisiana
- Ouray County, Colorado
- Outagamie County, Wisconsin
- Overton County, Tennessee
- Owen County, Indiana
- Owen County, Kentucky
- Owsley County, Kentucky
- Owyhee County, Idaho
- Oxford County, Maine
- Ozark County, Missouri
- Ozaukee County, Wisconsin
