1850 Atlas
| Name | What | Place | Pop | Other |
| Orland | PT | Hancock Co, ME | 1,579 | water by Penobscot river |
| Orland | PT | Steuben Co, IN | ||
| Orland | PO | Cook Co, IL | ||
| Orlean | PO | Fauquier Co, VA | ||
| Orleans | PT | Barnstable Co, MA | 1,848 | water by Cape Cod bay & Atlantic ocean |
| Orleans | T | Jefferson Co, NY | 3,265 | water by Chaumont & Perch rivers |
| Orleans | PV | Phelps twp, Ontario Co, NY | water by Flint creek | |
| Orleans | PO | Polk Co, MO | ||
| Orleans | PO | Alleghany Co, MD | ||
| Orleans | PV | Orange Co, IN | ||
| Orleans Four Corners | PO | Jefferson Co, NY | ||
| Orme's Store | PO | Bledsoe Co, TN | ||
| Orneville | PO | Piscataquis Co, ME | ||
| Orono | PT | Penobscot Co, ME | 2,785 | water by Penobscot & Pushaw rivers |
| Oronoco | PO | Amherst Co, VA | ||
| Oronoko | T | Berrien Co, MI | ||
| Orrin Glen | PO | Delaware Co, IA | ||
| Orrington | PT | Penobscot Co, ME | 1,852 | water by Penobscot river |
| Orrstown | PO | Franklin Co, PA | ||
| Orrsville | PV | Pine twp, Armstrong Co, PA | ||
| Orrville | PO | Mecklenburgh Co, NC | ||
| Orrville | PO | Dallas Co, AL | ||
| Orwell | PT | Rutland Co, VT | 1,682 | water by Lake Champlain, East creek & Lemonfair river |
| Orwell | PT | Oswego Co, NY | 1,106 | water by Salmon river |
| Orwell | PT | Ashtabula Co, OH | 825 | |
| Orwigsburgh | PB, CH | Schuylkill Co, PA | 909 | seat of justice, water by tributary of Schuylkill river |
| Osage | PO | Crawford Co, MO | ||
| Osage | T | Miller Co, MO | ||
| Osage | T | Morgan Co, MO | ||
| Osage | T | Van Buren Co, MO | ||
| Osage | PT | Carroll Co, AR | ||
| Osage | PO | Fremont Co, IA | ||
| Osanippa | PO | Chambers Co, AL | ||
| Osborn | PO | Rock Co, WI | ||
| Osborn Hollow | PO | Chenango twp, Broome Co, NY | ||
| Osborn's Bridge | PO | Northampton twp, Fulton Co, NY | ||
| Osborn's Ford | PO | Scott Co, VA | ||
| Osceola | PO | Lewis Co, NY | ||
| Oselicha | PO | Chambers Co, AL | ||
| Oseola | PV, CH | St Clair Co, MO | seat of justice, water by Osage river | |
| Oshaukuta | PO | Columbia Co, WI | ||
| Oshkosh | PO | Winnebago Co, WI | ||
| Oshtome | T | Kalamazoo Co, MI | 587 | |
| Oskaloosa | PV, CH | Mahaska Co, IA | 625 | seat of justice |
| Osnaburgh | PT | Stark Co, OH | 2,225 | |
| Ossawa | PO | Clinton Co, MI | ||
| Ossian | PT | Alleghany Co, NY | 1,283 | water by Canaseraga creek |
| Ossian | PO | Wells Co, IN | ||
| Ossipee | PT, CH | Carroll Co, NH | 2,123 | seat of justice, water by Ossipee lake & Bearcamp river |
| Ostend | PO | McHenry Co, IL | ||
| Ostend | PO | Washington Co, OH | ||
| Osterville | PV | Barnstable twp, Barnstable Co, MA | water by Oyster bay | |
| Oswayo | PO | Potter Co, PA | ||
| Oswegatchie | PT | St Lawrence Co, NY | 7,756 | water by Black lake , Oswegatchie & ST Lawrence rivers |
| Oswego | PV, CH | Oswego Co, NY | 12,205 | seat of justice w Pulaski, water by Oswego river, Lake Ontario & Oswego canal |
| Oswego | T | Potter Co, PA | ||
| Oswego | PV | Kosciusko Co, IN | 137 | |
| Oswego | PT | Kendall Co, IL | 1,599 | |
| Oswego Falls | PO | Oswego Co, NY | ||
| Oswego Village | PO | Dutchess Co, NY | ||
| Oswicchee | PO | Russell Co, AL | ||
| Otego | PT | Otsego Co, NY | 1,792 | water by Otsdawa creek & Susquehanna river |
| Otis | PT | Berhsire Co, MA | 1,224 | water by head waters of Farmington & Westfield rivers |
| Otis | T | Hancock Co, ME | 124 | |
| Otisco | PT | Onondaga Co, NY | 1,804 | water by Otisco lake & Nine Mile creek |
| Otisco | PT | Ionia Co, MI | 1,018 | |
| Otisfield | PT | Cumberland Co, ME | 1,171 | water by Crooked river |
| Otisville | PO | Mount Hope twp, Orange Co, NY | ||
| Otsdawa | PV | Otsego twp, Otsego Co, NY | ||
| Otsego | PT | Ostego Co, NY | 3,901 | water by Otsego & Canaderaga lakes, Oak creek |
| Otsego | PO | Muskingum Co, OH | ||
| Otsego | PO | Ray Co, MO | ||
| Otsego | PT | Allegan Co, MI | 818 | water by Kalamazoo river |
| Otsego | PO | Lake Co, IL | ||
| Otsego | PO | Columbia Co, WI | ||
| Otselic | PT | Chenango Co, NY | 1,800 | water by Ostelic creek |
| Ottawa | PV, CH | La Salle Co, IL | seat of justice, water by Illinois & Fox rivers | |
| Ottawa | PT | Ottawa Co, OH | ||
| Ottawa | T | Ottawa Co, MI | 430 | |
| Ottawa | T | Putnam Co, OH | 1,166 | |
| Ottawa Lake | PO | Monroe Co, MI | ||
| Otter Bridge | PO | Bedford Co, VA | ||
| Otter Creek | T | Ripley Co, IN | 741 | |
| Otter Creek | PO | Jersey Co, IL | ||
| Otter Creek | PO | Addison Co, VT | ||
| Otter Creek | PO | Jackson Co, MI | ||
| Otter Creek | T | Wayne Co, MO | ||
| Otter Dale | PO | Chesterfield Co, VA | ||
| Otter Village | PV | Ripley Co, IN | ||
| Otterville | PO | Cooper Co, MO | ||
| Otto | PO | Fulton Co, IL | ||
| Otto | PT | Cattaraugus Co, NY | 2,267 | water by Cattaraugus creek |
| Ottohee | PV | Fulton Co, MO | ||
| Ottobine | PO | Rockingham Co, VA | ||
| Ottsville | PO | Nockamixon twp, Bucks Co, PA | ||
| Ottuma | PV, CH | Wapello Co, IA | seat of justice | |
| Ouelout | PO | Delaware Co, NY | ||
| Oury's | PO | Hamilton Co, CT | ||
| Outlet | PO | Lake Co, IN | ||
| Overalls | PO | Warren Co, VA | ||
| Overfield | PO | Barbour Co, VA | ||
| Overman's Ferry | PO | Muscatine Co, IA | ||
| Ovid | PT | Branch Co, MI | 710 | |
| Ovid | PT, CH | Seneca Co, NY | 2,258 | seat of justice w Waterloo, water by Cayuga & Seneca lakes |
| Ovid | PO | Jefferson twp, Franklin Co, OH | ||
| Ovid | PV | Madison Co, IN | ||
| Ovid | PO | Lee Co, IL | ||
| Owasco | PT | Cayuga Co, NY | 1,254 | water by Owasco lake |
| Owasco Lake | PO | Cayuga Co, NY | ||
| Owascus | PO | Fond de Lac Co, WI | ||
| Owasso | PT | Shiawassee Co, MI | 392 | water by Shiawassee river |
| Owego | PT, CH | Tioga Co, NY | 7,159 | seat of justice, water by Owego creek & Susquehanna river |
| Owen | T | Dallas Co, AR | 366 | |
| Owensborough | PV, CH | Daviess Co, KY | 1,215 | seat of justice, water by Ohio river |
| Owensborough | PO | Green Co, IN | ||
| Owensville | V | Somers twp, Westchester Co, NY | water by Croton river | |
| Owensville | PO | Stonelick twp, Clermont Co, OH | ||
| Owensville | PV | Gibson Co, IN | ||
| Owensville | PV | Saline Co, AR | ||
| Owenton | PV, CH | Owen Co, KY | seat of justice | |
| Owenville | PV | Sampson Co, NC | ||
| Owing's Mills | PO | Baltimore Co, MD | ||
| Owingsville | PO | Anne Arundel Co, MD | ||
| Owingsville | PV, CH | Bath Co, KY | seat of justice, water by Slate creek | |
| Owl Prairie | PO | Daviess Co, IN | ||
| Oxbow | PV | Antwerp twp, Jefferson Co, NY | water by Oswegatchie river | |
| Oxbow | PO | Wyoming Co, PA | ||
| Oxbow | PO | Putnam Co, IL | ||
| Oxford | PT | Oxford Co, ME | 1,233 | water by Little Androscoggin river |
| Oxford | PT | Worcester Co, MA | 2,380 | water by French river |
| Oxford | PT | Chenango Co, NY | 3,227 | water by Chenango river & canal |
| Oxford | T | Warren Co, NJ | 1,718 | water by Pequest creek & Beaver brook |
| Oxford | PV | Chester Co, PA | 106 | |
| Oxford | T | Philadelphia Co, PA | 1,787 | water by Delaware river, Tacony & Sissisocksink creeks |
| Oxford | PT | Butler Co, OH | 3,139 | seat of Miami university |
| Oxford | T | Coshocton Co, OH | 1,112 | |
| Oxford | T | Delaware Co, OH | 828 | |
| Oxford | T | Guernsey Co, OH | 2,209 | |
| Oxford | T | Tuscarawas Co, OH | 1,436 | |
| Oxford | PT | Oakland Co, MI | 1,019 | |
| Oxford | PV, CH | Granville Co, NC | seat of justice, water by Fishing creek | |
| Oxford | PV | Henry Co, IL | ||
| Oxford | PV, CH | Lafayette Co, MS | seat of justice | |
| Oxford | PO | Newton Co, GA | ||
| Oxford | PT | New Haven Co, CT | 1,564 | water by Housatonic & Naugatuck rivers |
| Oxford | PV, CH | Benton Co, IN | seat of justice | |
| Oxford | PO | Scott Co, KY | ||
| Oxford | T | Erie Co, OH | 684 | |
| Oxford | PO | Talbot Co, MD | ||
| Oxford | PV | Grafton Co, NH | 1,406 | |
| Oxford | PO | Benton Co, AL | ||
| Oxford | PO | Chester Co, PA | ||
| Oxford | PO | McMinn Co, TN | ||
| Oxford | PO | Ritchie Co, VA | ||
| Oxford Depot | not listed | Orange Co, NY | ||
| Oxford Furnace | PV | Oxford twp, Warren Co, NJ | water by a tributary of Pequest creek | |
| Oxford Valley | PO | Bucks Co, PA | ||
| Oyster Bay | PT | Queen's Co, Long Island, NY | 6,900 | water by Oyster bay of Long Island sound & Great South bay of Atlantic ocean |
| Oyster Bay South | PV | Oyster Bay twp, Queen's Co, Long Island, NY | ||
| Oyster Pond | V | Southold twp, Suffolk Co, Long Island, NY | water by Oyster Pond harbor | |
| Ozan | T | Hempstead Co, AR | 1,547 | |
| Ozark | PV, CH | Franklin Co, AR | seat of justice, water by Arkansas river | |
| Ozark | PV | Greene Co, MO | 569 | |
| Ozark | V, CH | Ozark Co, MO | seat of justice, water by north fork of White river | |
| Ozaukee | PO | Washington Co, WI |