List of all counties in ABC order, that begin on letter O. Counties with red border around name have population of more than 100 000. Double width border represents the population of 500 000+.
O'Brien County, IA Oakland County, MI Obion County, TN Ocean County, NJ Oceana County, MI Ochiltree County, TX Oconee County, GA Oconee County, SC Oconto County, WI Ogemaw County, MI Oglala Lakota County, SD Ogle County, IL Oglethorpe County, GA Ohio County, IN Ohio County, KY Ohio County, WV Okaloosa County, FL Okanogan County, WA Okeechobee County, FL Okfuskee County, OK Oklahoma County, OK Okmulgee County, OK Oktibbeha County, MS Oldham County, KY Oldham County, TX Oliver County, ND Olmsted County, MN Oneida County, ID Oneida County, NY Oneida County, WI Onondaga County, NY Onslow County, NC Ontario County, NY Ontonagon County, MI Orange County, CA Orange County, FL Orange County, IN Orange County, NC Orange County, NY Orange County, TX Orange County, VA Orange County, VT Orangeburg County, SC Oregon County, MO Orleans County, NY Orleans County, VT Orleans Parish, LA Osage County, KS Osage County, MO Osage County, OK Osborne County, KS Osceola County, FL Osceola County, IA Osceola County, MI Oscoda County, MI Oswego County, NY Otero County, CO Otero County, NM Otoe County, NE Otsego County, MI Otsego County, NY Ottawa County, KS Ottawa County, MI Ottawa County, OH Ottawa County, OK Otter Tail County, MN Ouachita County, AR Ouachita Parish, LA Ouray County, CO Outagamie County, WI Overton County, TN Owen County, IN Owen County, KY Owsley County, KY Owyhee County, ID Oxford County, ME Ozark County, MO Ozaukee County, WI
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