| Inscription | Country | Years | #Issued |
| Obock | Obock | 1892-94 | 90 |
| Occussi-Ambeno | Bogus | ||
| Oesterreich Post | Austria | 1890-04 | |
| Offentlig Sak | Norway (official) | 1933-49 | 50 |
| Off. Sak | Norway (official) | 1855- | 50 |
| OKCA | Russia: Army of the North | 1919 | 5 |
| Oldenburg | German States: Oldenburg(sta) | 1852-67 | 30 |
| Oltre Giuba(ovpt) | Italy: Jubaland(English before 1924) | 1925-26 | 80 |
| Orange River Colony | Orange River Colony | 1903-08 | 15 |
| Oranje Vrij Staat | Orange River Colony | 1868-1902 | 100 |
| Orchha | India: Orchha(sta) | 1913-40 | 20? |
| Orcha | India: Orchha(sta) | 1913-40 | 7? |
| Orts Post | Switzerland | 1850 | 2 |
| O.S. | Norway (official) | 1951-52 | 7 |
| Osterreich | Austria | 1922-45 | |
| Osterreichische Post | Austria | 1908-19 | |
| Oubangui-Chari | Ubangi-Shari | 1915-33 | 110 |
If there are any missing inscription, tell me : marcosampaio@grupo.bfe.pt
