1850 Atlas
| Name | What | Place | Pop | Other |
| Oasis | PO | Marquette Co, WI | ||
| Oatlands | PO | Loudon Co, VA | ||
| Oberlin | PV | Russia twp, Lorain Co, OH | seat of Oberlin Collegiate Institute | |
| Obion | PO | Hickman Co, KY | ||
| Obispo | PO | San Louis Obispo Co, CA | ||
| Occoquan | PV | Prince William Co, VA | ||
| Occupacia | PO | Essex Co, VA | ||
| Ocean Port | PO | Monmouth Co, NJ | ||
| Ocela | PO | Cherokee Co, AL | ||
| Ocela | PT | Livingston Co, MI | ||
| Ocela | PO | Crawford Co, OH | ||
| Ocela | PV, CH | Mississippi Co, AR | seat of justice, water by Mississippi river | |
| Ocela Centre | PV | Ocela twp, Livingston Co, MI | ||
| Ochessee | PO | Calhoun Co, FL | ||
| Ocklockney | PO | Thomas Co, GA | ||
| Ocmulgee | PO | Perry Co, AL | ||
| Ocumlgeeville | PV | Telfair Co, GA | ||
| Ocoa | PO | Polk Co, TN | ||
| Ocola | PO | Marion Co, FL | ||
| Ocona Lufty | PO | Haywood Co, NC | ||
| Oconee | PO | Washington Co, GA | ||
| Oconee Station | PO | Pickens dist, SC | ||
| Oconmowock | PO | Waukesha Co, WI | ||
| Ocracoke | PV | Hyde Co, NC | water by Ocracoke inlet | |
| Octoraro | PO | Lancaster Co, PA | ||
| Oddville | PO | Harrison Co, KY | ||
| Oenca | PO | Jefferson Co, WI | ||
| Ogalla | PO | Chippewa Co, WI | ||
| Ogden | PT | Monroe Co, NY | 2,598 | water by tributaries of Salmon & Rush creeks, Erie canal |
| Ogden | PV | Henry Co, IN | ||
| Ogden | T | Lenawee Co, MI | 579 | |
| Ogden | PO | New Madrid Co, MO | ||
| Ogdensburgh | PV | Oswegatchie twp, St Lawrence Co, NY | water by St Lawrence river at the confluence of the Oswegatchie | |
| Ogdensburgh | PO | Union Co, PA | ||
| Ogeechee | PO | Scriven Co, GA | ||
| Ogle | PO | Butler Co, PA | ||
| Ogle | PO | Ogle Co, IL | ||
| Oglethorpe | PO | Macon Co, GA | ||
| Ogunquit | PO | York Co, ME | ||
| Ohio | PT | Herkimer Co, NY | 1,051 | water by West Camden creek |
| Ohio | T | Alleghany Co, PA | 2,329 | water by Ohio river, Great & Little Sewickly creeks; Johnes & Hawser's runs |
| Ohio | T | Beaver Co, PA | 1,660 | water by Little Beaver creek & Ohio river |
| Ohio | T | Clermont Co, OH | 4,479 | |
| Ohio | T | Gallia Co, OH | 504 | water by Ohio river |
| Ohio | PT | Monroe Co, OH | 1,444 | |
| Ohio | T | Warwick Co, IN | 924 | |
| Ohio | T | Boones Co, IL | ||
| Ohio | PO | Andrew Co, MO | ||
| Ohio City | PV | Brooklyn twp, Cuyahoga Co, OH | water by Lake Erie, at the mouth of Cuyahoga river | |
| Ohio City | PO | Mississippi Co, MO | ||
| Ohio Farm | PO | Kendall Co, IL | ||
| Ohio Grove | PO | De Kalb Co, IL | ||
| Ohioville | PV | Ohio twp, Beaver Co, PA | ||
| Ohlstown | PO | Trumbull Co, OH | ||
| Oil Creek | PT | Crawford Co, PA | 811 | water by Oil creek |
| Oil Mills | PO | Clarke Co, KY | ||
| Oil Mills Village | PO | Hillsborough Co, NH | ||
| Oil Trough | PO | Independence Co, AR | ||
| Okahumka | PO | Marion Co, FL | ||
| Okauchee | PO | Waukesha Co, WI | ||
| Okapilco | PO | Lowndes Co, GA | ||
| Okaw | PO | Washington Co, IL | ||
| Okolona | PO | Chickasaw Co, MS | ||
| Olamon | PO | Penobscot Co, ME | ||
| Olcott | PV | Newfane twp, Niagara Co, NY | water by Lake Ontario & Eighteen Mile creek | |
| Old Bridge | PV | North Berwick twp, Middlesex Co, NJ | ||
| Old Cedar Springs | PO | Spartanburgh dist, SC | ||
| Old Church | PO | Hanover Co, VA | ||
| Old Court House | PO | Bryan Co, GA | ||
| Old Codorus | T | York Co, PA | water by Codorus creek | |
| Old Farm | PO | Lawrence Co, IL | ||
| Old Forge | PO | Luzerne Co, PA | ||
| Old Fort | PO | Centre Co, PA | ||
| Old Fort | PO | McDowell Co, NC | ||
| Old Franklin | PO | Howard Co, MO | ||
| Old Furnace | PO | Gaston Co, NC | ||
| Old Hickory | PO | Bradford Co, PA | ||
| Old Hickory | PO | Wayne Co, OH | ||
| Old Hickory | PO | Simpson Co, MS | ||
| Old Hickory | PO | Botetourt Co, VA | ||
| Old Hickory | PO | Weakley Co, TN | ||
| Old Mines | PV | Washington Co, MO | ||
| Old Mission | PO | Winneshiek Co, IA | ||
| Old Point Comfort | PO | Elizabeth City Co, VA | situated on James river | |
| Old River | T | Arkansas Co, AR | ||
| Old River | T | Chicot Co, AR | 867 | |
| Old Town | PT | Penobscot Co, ME | 3,087 | water by Penobscot river |
| Old Town | PV | Alleghany Co, MD | water by Potomac river | |
| Old Town | PV | Forsyth Co, NC | ||
| Old Town | PV | Philips Co, AR | ||
| Old Town | PV | Claiborne Co, TN | ||
| Old Town | PO | Greenup Co, KY | ||
| Old Town | PO | Coffee Co, AL | ||
| Old Washington | PV, CH | Washington Co, AL | seat of justice | |
| Oldenburgh | PO | Franklin Co, IN | ||
| Oldenburgh | PO | Smith Co, TX | ||
| Oldfield Fork of Elk | PO | Pocahontas Co, VA | ||
| Oldham | PO | Crittenden Co, AR | ||
| Oldham Cross Roads | PO | Westmoreland Co, VA | ||
| Olean | PT | Cattaraugus Co, NY | 899 | water by Alleghany river & Genesee River canal |
| Olean | PO | Ripley Co, IN | ||
| Oleander | PO | Marshall Co, AL | ||
| Olena | PO | Henderson Co, IL | ||
| Olena | PO | Huron Co, OH | ||
| Olentangy | PO | Crawford Co, OH | ||
| Oley | PT | Berks Co, PA | water by head waters of Manatawny & Manooskisy creeks | |
| Olinda | PO | Fayette Co, AL | ||
| Olio | PO | Hamilton Co, IN | ||
| Olio | PO | Stoddard Co, MO | ||
| Olive | PT | Ulster Co, NY | 2,710 | water by Esopus creek |
| Olive | PT | Morgan Co, OH | 2,013 | |
| Olive | T | Meigs Co, OH | 924 | water by Shade river |
| Olive | PO | Clinton Co, MI | ||
| Olive | PO | Marion Co, IN | ||
| Olive Branch | PO | De Soto Co, MS | ||
| Olive Branch | PO | Clermont Co, OH | ||
| Olive Branch | PO | Holt Co, MO | ||
| Olive Bridge | PO | Ulster Co, NY | ||
| Olive Grove | PO | Decatur Co, GA | ||
| Olive Hill | PO | Carter Co, KY | ||
| Olive Hill | PO | Person Co, NC | ||
| Oliver | T | Mifflin Co, PA | 1,688 | |
| Oliver | T | Perry Co, PA | ||
| Oliver's | PO | Anderson Co, TN | ||
| Oliver's Prairie | PO | Newton Co, MO | ||
| Olivesburgh | PV | Milton twp, Richland Co, OH | water by Whetstone river | |
| Olivet | PO | Eaton Co, MI | ||
| Olivet | PO | Armstrong Co, PA | ||
| Olivet | PO | Russell Co, AL | ||
| Olmstead | PT | Cuyahoga Co, OH | 1,216 | |
| Olney | PO | Montgomery Co, MD | ||
| Olney | PV, CH | Richland Co, IL | seat of justice, water by Little Wabash river | |
| Olney | PO | Pickens Co, AL | ||
| Olney | PO | Philadelphia Co, PA | ||
| Olneyville | PO | Providence Co, RI | ||
| Olustee | PO | Columbia Co, FL | ||
| Olustee Creek | PO | Pike Co, AL | ||
| Olympian Springs | PO | Bath Co, KY | ||
| Olympus | T | Muskingum Co, OH | ||
| Plympus | PO | Overton Co, TN |