Feature Name | Class | Elev. meters | County | Latitude DMS | Longitude DMS |
| Old Saint Josephs Cemetery | Cemetery | 30 | St. Mary's | 38 22 29 N | 076 41 35 W |
| Old Saint Lukes Cemetery | Cemetery | 12 | Caroline | 38 53 06 N | 075 49 50 W |
| Old Saint Martins Cemetery | Cemetery | 6 | Worcester | 38 23 35 N | 075 12 18 W |
| Old Saint Martins Church | Church | 6 | Worcester | 38 23 34 N | 075 12 16 W |
| Old Salem Village | Populated Place | 143 | Montgomery | 39 05 37 N | 077 00 21 W |
| Old School Baptist Church | Church | 8 | Wicomico | 38 22 01 N | 075 35 54 W |
| Old School Baptist Church | Church | 6 | Worcester | 38 10 29 N | 075 23 28 W |
| Old Severna Park | Populated Place | 13 | Anne Arundel | 39 04 13 N | 076 33 09 W |
| Old Shiloh Cemetery | Cemetery | 2 | Worcester | 38 06 16 N | 075 29 17 W |
| Old South Country Club | Locale | 46 | Anne Arundel | 38 49 05 N | 076 39 19 W |
| Old Swamp Schoolhouse (historical) | School | 11 | Worcester | 38 24 01 N | 075 18 15 W |
| Old Sycamore Estates | Populated Place | 50 | Charles | 38 26 57 N | 076 52 07 W |
| Old Town | Populated Place | 6 | Baltimore (city) | 39 17 38 N | 076 36 26 W |
| Old Town | Populated Place | 6 | Kent | 39 08 31 N | 076 06 22 W |
| Old Town Medical Center | Hospital | 12 | Baltimore (city) | 39 17 52 N | 076 36 02 W |
| Old Town Point Wharf | Locale | 0 | Cecil | 39 30 09 N | 075 54 57 W |
| Old Trinity Cemetery | Cemetery | 11 | Worcester | 38 14 10 N | 075 18 29 W |
| Old Trinity Church | Church | 0 | Dorchester | 38 28 16 N | 076 18 29 W |
| Old Villa | Populated Place | 4 | Talbot | 38 47 54 N | 076 06 33 W |
| Old Wharf Landing | Locale | 5 | St. Mary's | 38 15 14 N | 076 47 21 W |
| Old Whitemarsh Church (historical) | Church | 16 | Talbot | 38 41 48 N | 076 03 38 W |
| Old Womans Cove | Bay | 0 | Anne Arundel | 38 58 21 N | 076 29 55 W |
| Old Womans Gut | Stream | | Harford | 39 24 19 N | 076 09 02 W |
| Old Womans Run | Stream | | Charles | 38 35 46 N | 077 03 16 W |
| Old Wye Church | Church | 12 | Talbot | 38 56 11 N | 076 04 47 W |
| Old York Manor | Populated Place | 171 | Baltimore | 39 30 57 N | 076 34 36 W |
| Old York Manor | Populated Place | 210 | Harford | 39 38 55 N | 076 33 44 W |
| Olde Forte Village Shopping Center | Locale | 15 | Prince George's | 38 43 33 N | 076 59 29 W |
| Olde Mill | Populated Place | 39 | Calvert | 38 27 47 N | 076 34 00 W |
| Olde Mill Estates | Populated Place | 169 | Baltimore | 39 22 57 N | 076 46 27 W |
| Olde Oak Farm | Locale | 37 | Charles | 38 24 54 N | 076 51 22 W |
| Olde Oak Run | Populated Place | 218 | Carroll | 39 25 11 N | 077 07 16 W |
| Olde Severna Park Professional Center | Locale | | Anne Arundel | 39 04 10 N | 076 32 40 W |
| Olde Washington Valley | Populated Place | 245 | Carroll | 39 27 41 N | 077 00 37 W |
| Oldfield | Populated Place | 25 | Calvert | 38 35 55 N | 076 31 29 W |
| Oldfield | Populated Place | 186 | Frederick | 39 29 46 N | 077 11 34 W |
| Oldfield Acres | Populated Place | 16 | Cecil | 39 33 13 N | 075 52 43 W |
| Oldfield Branch | Stream | | Frederick | 39 28 18 N | 077 13 09 W |
| Oldfield Point | Cape | 1 | Cecil | 39 30 44 N | 075 54 07 W |
| Oldfield School | School | 124 | Baltimore | 39 33 19 N | 076 37 31 W |
| Oldtown | Populated Place | 174 | Allegany | 39 32 29 N | 078 36 41 W |
| Oldtown Baptist Church | Church | 175 | Allegany | 39 32 30 N | 078 36 44 W |
| Oldtown Branch | Stream | | Caroline | 39 01 22 N | 075 46 38 W |
| Oldtown Catholic Cemetery | Cemetery | 198 | Allegany | 39 32 40 N | 078 36 52 W |
| Oldtown K-12 School | School | 170 | Allegany | 39 32 28 N | 078 36 31 W |
| Oldtown Post Office | Post Office | 171 | Allegany | 39 32 28 N | 078 36 41 W |
| Oldtown United Methodist Cemetery | Cemetery | 193 | Allegany | 39 32 39 N | 078 36 49 W |
| Oldtown Volunteer Fire Department | Building | | Allegany | 39 54 37 N | 079 02 20 W |
| Oldtown-Battie Mixon Picnic Area | Locale | 172 | Allegany | 39 32 34 N | 078 37 10 W |
| Olive | Populated Place | 135 | Frederick | 39 19 39 N | 077 34 57 W |
| Olive Branch Lodge Park | Park | 11 | Caroline | 38 53 13 N | 075 49 46 W |
| Olive Branch United Methodist Church | Church | 10 | Baltimore (city) | 39 16 20 N | 076 36 52 W |
| Olive School (historical) | School | 135 | Frederick | 39 19 39 N | 077 35 04 W |
| Oliver Beach | Populated Place | 3 | Baltimore | 39 22 22 N | 076 20 34 W |
| Oliver Beach Elementary School | School | 6 | Baltimore | 39 22 12 N | 076 21 15 W |
| Oliver Co Dam | Dam | 233 | Washington | 39 43 18 N | 077 53 53 W |
| Oliver Cromwell School | School | 40 | Baltimore (city) | 39 18 53 N | 076 36 26 W |
| Oliver Perry Public School (historical) | School | 8 | Baltimore (city) | 39 16 30 N | 076 36 50 W |
| Oliver Point | Cape | 2 | Baltimore | 39 22 25 N | 076 20 17 W |
| Olivers Grove Church | Church | 286 | Allegany | 39 35 08 N | 078 39 57 W |
| Olivers Shop | Locale | 56 | Charles | 38 28 56 N | 076 52 23 W |
| Olivet | Populated Place | 3 | Calvert | 38 20 16 N | 076 26 34 W |
| Olivet Cemetery | Cemetery | 21 | Kent | 39 20 19 N | 075 52 37 W |
| Olivet Cemetery | Cemetery | 13 | Worcester | 38 12 52 N | 075 33 42 W |
| Olivet Church | Church | 21 | Kent | 39 20 18 N | 075 52 38 W |
| Olivet Church | Church | 11 | Worcester | 38 12 46 N | 075 33 29 W |
| Olivet Hill | Populated Place | 19 | Kent | 39 20 17 N | 075 51 58 W |
| Olivet School | School | 27 | Calvert | 38 21 40 N | 076 26 14 W |
| Olivet United Methodist Church | Church | 3 | Calvert | 38 20 12 N | 076 26 36 W |
| Olmsted Island | Island | 44 | Montgomery | 38 59 55 N | 077 15 03 W |
| Olne Acres Poultry Farm (historical) | Locale | 147 | Montgomery | 39 06 01 N | 076 56 24 W |
| Olney | Populated Place | 165 | Montgomery | 39 09 11 N | 077 04 01 W |
| Olney Acres | Populated Place | 133 | Montgomery | 39 09 00 N | 077 05 29 W |
| Olney Baptist Church | Church | 167 | Montgomery | 39 08 39 N | 077 04 01 W |
| Olney Church of Christ | Church | 167 | Montgomery | 39 08 08 N | 077 04 11 W |
| Olney Elementary School | School | 171 | Montgomery | 39 08 57 N | 077 04 04 W |
| Olney Estates | Populated Place | 142 | Montgomery | 39 08 48 N | 077 02 50 W |
| Olney Library | Building | 161 | Montgomery | 39 09 17 N | 077 04 10 W |
| Olney Library | Building | 164 | Montgomery | 39 09 14 N | 077 04 08 W |
| Olney Mill | Populated Place | 141 | Montgomery | 39 10 33 N | 077 04 19 W |
| Olney Oaks | Populated Place | 143 | Montgomery | 39 09 09 N | 077 04 53 W |
| Olney Post Office | Post Office | 153 | Montgomery | 39 09 18 N | 077 04 18 W |
| Olney Shopping Center | Locale | 165 | Montgomery | 39 09 11 N | 077 04 07 W |
| Olney Square | Populated Place | 138 | Montgomery | 39 09 29 N | 077 04 54 W |
| Olney Square Park | Park | 139 | Montgomery | 39 09 28 N | 077 04 49 W |
| Olney Theater | Building | 154 | Montgomery | 39 08 41 N | 077 02 39 W |
| Olney Town | Populated Place | 162 | Montgomery | 39 09 28 N | 077 04 05 W |
| Omar (historical) | Populated Place | 23 | Anne Arundel | 39 03 47 N | 076 35 44 W |
| Omar Acres | Populated Place | 203 | Carroll | 39 32 14 N | 077 01 15 W |
| One Hundred Forty Village Shopping Center | Locale | 275 | Carroll | 39 33 52 N | 076 58 41 W |
| Onion Prall Laboratory Preschool | Building | | Frederick | 39 42 06 N | 077 42 14 W |
| Open Acres Manor | Populated Place | 90 | Harford | 39 29 26 N | 076 22 19 W |
| Open Bible Academy | School | 4 | Dorchester | 38 33 04 N | 076 05 06 W |
| Open Bible Church | Church | 4 | Dorchester | 38 33 03 N | 076 05 06 W |
| Open Bible Gospel Tabernacle | Church | 100 | Baltimore (city) | 39 21 21 N | 076 33 32 W |
| Open Bible Tabernacle | Church | 149 | Howard | 39 17 52 N | 076 48 39 W |
| Open Door School | School | 118 | Montgomery | 39 02 52 N | 077 01 38 W |
| Opequon Farms | Populated Place | 134 | Washington | 39 32 53 N | 077 50 22 W |
| Opequon Junction Campground | Locale | 114 | Washington | 39 31 08 N | 077 51 49 W |
| Opossum Hill | Summit | 18 | Caroline | 38 47 09 N | 075 48 23 W |
| Opossum Island | Island | 0 | Dorchester | 38 19 50 N | 076 14 58 W |
| Orange Grove (historical) | Populated Place | 96 | Baltimore | 39 14 30 N | 076 43 56 W |
| Orange Grove (historical) | Populated Place | 9 | Howard | 39 14 34 N | 076 45 03 W |
| Orange Grove Scenic Area | Park | 64 | Howard | 39 14 05 N | 076 44 44 W |
| Orangeville | Populated Place | 21 | Baltimore (city) | 39 17 55 N | 076 32 59 W |
| Orangeville Methodist Church | Church | 31 | Baltimore (city) | 39 18 09 N | 076 33 44 W |
| Oraville | Populated Place | 49 | St. Mary's | 38 25 04 N | 076 41 39 W |
| Oraville Post Office (historical) | Post Office | 49 | St. Mary's | 38 25 04 N | 076 41 42 W |
| Orchard Beach | Populated Place | 12 | Anne Arundel | 39 10 14 N | 076 31 36 W |
| Orchard Beach Fire Company 11 | Building | | Anne Arundel | 39 10 42 N | 076 33 02 W |