| City Name | County | Zip Code |
| Oakland | Garrett | 21550 |
| Ocean City | Worcester | 21842 |
| Ocean Pines | Worcester | 21811 |
| Odenton | Anne Arundel | 21113 |
| Oldtown | Allegany | 21555 |
| Olivet Hill | Kent | 21637 |
| Olney | Montgomery | ***** |
| Orchard Beach | Anne Arundel | 21226 |
| Oriole | Somerset | 21853 |
| Overlea | Baltimore | 21206 |
| Owings | Calvert | 20736 |
| Owings Mills | Baltimore | 21117 |
| Oxford | Talbot | 21654 |
| Oxon Hill | Prince George's | ***** |
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