1850 Atlas
| Name | What | Place | Pop | Other |
| Omar | PV | Villenova twp, Jefferson Co, NY | water by west branch of Conewango creek | |
| Omega | PO | Pike Co, OH | ||
| Omro | PO | Winnebago Co, WI | ||
| Onancock | PV | Accomac Co, VA | water by Chesapeake bay | |
| O'Neal's Mills | PO | Troup Co, GA | ||
| Onecho | PO | Montgomery Co, TN | ||
| Oneco | PO | Stephenson Co, IL | ||
| Oneida | PT | Eaton Co, MI | 492 | |
| Oneida | PO | Brown Co, WI | ||
| Oneida Castle | PV | Vernon twp, Oneida Co, NY | water by Oneida creek | |
| Oneida Depot | PO | Verona twp, Madison Co, NY | ||
| Oneida Lake | PO | Lenox twp, Madison Co, NY | ||
| Oneida Mills | PO | Carroll Co, OH | ||
| Oneida Valley | PO | Madison Co, NY | ||
| Oneonta | PT | Otsego Co, NY | 1,903 | water by Susquehanna river |
| Onion River | PO | Sheboygan Co, WI | ||
| Onondaga | PT | Onondaga Co, NY | 5,694 | water by Onondaga creek |
| Onondaga | PO | Ingham Co, MI | ||
| Onondaga | PO | Marshall Co, IN | ||
| Onondaga Castle | PO | Onondaga Co, NY | ||
| Onondaga Hollow | V | Onondaga twp, Onondaga Co, NY | ||
| Onondaga Valley | PO | Onondaga Co, NY | ||
| Onoville | PO | Cattaraugus Co, NY | ||
| Onslow | PV, CH | Onslow Co, NC | seat of justice, water by New river | |
| Ontario | PT | Wayne Co, NY | 2,246 | water by Lake Ontario |
| Ontario | PO | Richland Co, OH | ||
| Ontario | PO | La Grange Co, IN | ||
| Ontario | PO | Knox Co, IL | ||
| Ontonagon | PO | Ontonagon Co, MI | ||
| Ontwa | T | Cass Co, MI | 780 | |
| Onward | PO | Stewart Co, TN | ||
| Oosoola | PO | Somerset Co, ME | ||
| Oothcaloga | PO | Cass Co, GA | ||
| Opalika | PO | Walker Co, GA | ||
| Opelika | PO | Russell Co, AL | ||
| Opelousas | PV, CH | St Landry parish, LA | seat of justice, water by Vermilion river, seat of Franklin College | |
| Open Pond | PO | Henry Co, AL | ||
| Ophir | PO | La Salle Co, IL | ||
| Oporto | PO | St Joseph Co, MI | ||
| Oppenheim | PT | Fulton Co, NY | 2,315 | water by East Canada creek |
| Oquawka | PV | Henderson Co, IL | seat of justice, water by Mississippi river | |
| Oran | PV | Pomepy twp, Onondaga Co, NY | ||
| Oran | PO | Kosciusko Co, IN | ||
| Orange | PT | Orange Co, VT | 1,007 | water by Jail branch of Onion river |
| Orange | T | Grafton Co, NH | 451 | |
| Orange | PT | Franklin Co, MA | 1,701 | water by Miller's river |
| Orange | T | New Haven Co, CT | 1,476 | water by Wopewaug river |
| Orange | PT | Steuben Co, NY | 2,055 | water by Mead's creek |
| Orange | PT | Essex Co, NJ | 4,385 | water by Second river |
| Orange | PT | Columbia Co, PA | ||
| Orange | T | Cuyahoga Co, OH | 1,063 | water by Chagrin river |
| Orange | T | Delaware Co, OH | 1,150 | |
| Orange | T | Hancock Co, OH | 704 | |
| Orange | T | Meigs Co, OH | 948 | |
| Orange | PT | Richland Co, OH | ||
| Orange | T | Shelby Co, OH | 922 | |
| Orange | PO | Austintown twp, Mahoning Co, OH | ||
| Orange | T | Noble Co, IN | 607 | |
| Orange | T | Rush Co, IN | ||
| Orange | T | Ionia Co, MI | 378 | |
| Orange | PV | Cherokee Co, GA | ||
| Orange | PT | Fayette Co, IN | 1,129 | |
| Orange | PO | Luzerne Co, PA | ||
| Orange | PT | Columbia Co, PA | 1,077 | |
| Orange | PV, CH | Orange Co, VA | seat of justice | |
| Orange Hill | PO | Washington Co, FL | ||
| Orange Lake | PO | Marion Co, FL | ||
| Orange Port | PO | Niagara Co, NY | ||
| Orange Springs | PV | Orange Co, VA | ||
| Orange Springs | PO | Marion Co, FL | ||
| Orangeburgh | CH | Orangeburgh Dist, SC | seat of justice, water by North Edisto river | |
| Orangeburgh | PO | Mason Co, KY | ||
| Orangetown | T | Rockland Co, NY | 4,769 | water by Hudson & Hackensack rivers |
| Orangeville | PT | Wyoming Co, NY | 1,438 | water by Tonawanda creek |
| Orangeville | PV | Bloom twp, Columbia Co, PA | water by Fishing creek | |
| Orangeville | PO | Trumbull Co, OH | ||
| Orbisonia | PO | Huntingdon Co, PA | ||
| Orcutt Creek | PO | Bradford Co, PA | ||
| Orefield | PO | Lehigh Co, PA | ||
| Oregon | PO | Jefferson Co, AL | ||
| Oregon | PO | Lancaster Co, PA | ||
| Oregon | PV, CH | Holt Co, MO | seat of justice, water by Missouri river | |
| Oregon | T | Lucas Co, OH | 436 | |
| Oregon | PO | Clark Co, IN | ||
| Oregon | PO | Stocton twp, Chautauque Co, NY | ||
| Oregon | PO | Dane Co, WI | ||
| Oregon | PV | Ogle Co, IL | 540 | water by Rock river |
| Oregon | PO | Lincoln Co, TN | ||
| Oregonia | PO | Tuscaloosa Co, AL | ||
| Ore Hill | PO | Litchfield Co, CT | ||
| Orford | PT | Grafton Co, NH | 1,406 | water by Connecticut river |
| Orfordville | PV | Orford twp, Grafton Co, NH | ||
| Organ Church | PO | Rowan Co, NC | ||
| Orient | PT | Aroostook Co, ME | 205 | |
| Orient | PV | Southold twp, Suffolk Co, Long Island, NY | water by Oyster Pond harbor | |
| Orion | T | Oakland Co, MI | 1,119 | water by Paint creek |
| Orion | PO | Pike Co, AL | ||
| Orion | PO | Richland Co, WI | ||
| Orion | PO | Henry Co, IL | ||
| Oriskany | PO | Whitestown twp, Oneida Co, NY | ||
| Oriskany Falls | PV | Augusta twp, Oneida Co, NY | water by Oriskany creek & Chenango canal | |
| Orizaba | PO | Tippah Co, MS |