| Record Number | Keyword(s) | Country | Catalog Listed? | Continent | |
| 1737 | O. A. (overprint) | South Australia, officials (1868-74) [Official Assignee]. | Real | Australia |  |
| 1738 | O.B. (overprint) | Philippines, officials (1906-present) [Official Business]. | Real | Asia | |
| 1739 | O.F. CASTELLORISO (overprint on stamps of France) | France- Castellorizo occupation. | Real | Europe |  |
| 1740 | O.H.E.M.S. (overprint) | Egypt, officials (1922). | Real | Africa | |
| 1741 | O.H.H.S. (overprint) | Egypt, officials (1907-1922) [On His Highness’ Service]. | Real | Africa | |
| 1742 | O.H.M.S. (overprint) | Canada, officials [On His (or Her) Majestys Service]. | Real | Americas | |
| 1743 | O.K.C.A. POCCIЯ | Russia- Army of the North (1918). | Real | Asia |  |
| 1744 | O.L. (overprint) | Monaco, letters handed to mailmen or found in post boxes prior to 1908 [Origine Locale (Local Origin)]. | Real | Europe | |
| 1745 | O.M.F. CILICIE (overprint on stamps of France) | France- occupation of Cilicia. | Real | Europe |  |
| 1746 | O.M.F. SYRIE (overprint on stamps of France) | Syria (1920-22) [Occupation Militaire Francaise (French Military Occupation)]. | Real | Asia |  |
| 1747 | O.N.F. CASTELLORIZO (overprint on stamps of French Levant) | France- Castellorizo occupation [Occupation Navale Francaise (French Naval Occupation)]. | Real | Europe |  |
| 1748 | O.P.S.O. (overprint) | New Zealand, officials for Post Office sent abroad [On Public Service Only]. | Real | Australia | |
| 1749 | O.S. | Norway, officials (1951-2) [Offentlig Sax]. | Real | Europe | |
| 1750 | O.S. | English language countries, officials [On Service]. | Real | Europe | |
| 1751 | O.S.G.S. (overprint) | Sudan, officials [On Sudan Government Service]. | Real | Africa | |
| 1752 | O.T. (overprint) | Czechoslovakia, newspaper stamps used by commercial firms. | Real | Europe | |
| 1753 | O.U.S. (overprint between wavy lines) | Great Britain, official overprint for the Oxford Union Society to prevent pilfering of 1d reds. | Real | Europe |  |
| 1754 | O.W. OFFICIAL (overprint) | Great Britain- Office of Public Works, officials (1896-1902). | Real | Europe |  |
| 1755 | O.Ф. (overprint with airplane) | Bulgaria, airmails. | Real | Europe |  |
| 1756 | OAHA MAPKA | Finland (1866-74). | Real | Europe |  |
| 1757 | OAXACA | Mexico- Oaxaca (1914). | Real | Americas |  |
| 1758 | OBOCK (overprint on stamps of French Colonies) | Obock. | Real | Asia |  |
| 1759 | OCCUPATION AZIRBAYEDJAN (overprint on stamps of Russia) | Azerbaijan, unofficial overprints by Entente Allied officers (1918). | Real | Europe |  |
| 1760 | OCCUPATION FRANCAISE (overprint) | Hungary- French occupation (1919). | Real | Europe |  |
| 1761 | OCCUPATION FRANCAISE CAMEROUN (overprint on stamps of Middle Congo) | Cameroon- French occupation. | Real | Africa |  |
| 1762 | OCCUSSI-AMBENO | Bogus stamps for part of the island of Portuguese Timor. | Not Scott Listed | none |  |
| 1763 | OCEAN POSTAGE | Great Britain Penny Postage cutouts from envelopes during the 1850's. | Not Scott Listed | Europe |  |
| 1764 | OCEANIE | French Polynesia (1892-1958). | Real | Australia |  |
| 1765 | OCUPACION DE PASQUA | Chile, semipostal for Easter Island. | Real | Americas | |
| 1766 | OENEO ISLAND, DEPENDENCY OF | Bogus overprint of Pitcairn Island. | Not Scott Listed | Australia | |
| 1767 | OESTERR | Austria (1883-1907) [Austria]. | Real | Europe | |
| 1768 | OESTERR POST LIECHTENSTEIN | Liechtenstein- Austrian postal administration (1912-20). | Real | Europe | |
| 1769 | OESTERR POST with KAIS KOENIGL | Austria (1883-1907). | Real | Europe |  |
| 1770 | OEUVRES DE GUERRE (overprint) | Cameroon, semipostals. | Real | Africa | |
| 1771 | OEUVRES DE SOLIDARITE FRANCAISE | French Colonies, semipostals (1943-4). | Real | Africa |  |
| 1772 | OEUVRES SOCIALES (overprint on stamps of St. Pierre & Miquelon) | St. Pierre & Miquelon, semipostals. | Real | Americas |  |
| 1773 | OFF. SAK. | Norway, officials. | Real | Europe |  |
| 1774 | OFFENTLIG SAK | Norway, officials. | Real | Europe |  |
| 1775 | OFFICIAL (overprint on stamps of Kenya and Uganda) | Tanganyika, officials (1959-60). | Real | Africa |  |
| 1776 | OFFICIEL (overprint) | Luxembourg, officials. | Real | Europe | |
| 1777 | OFFICIEL (overprint) | Iran, officials denoting genuiness (1885-7). | Real | Asia | |
| 1778 | OFFISIEEL | South West Africa, officials. | Real | Africa | |
| 1779 | OFFISIEEL-OFFICIAL | South Africa, officials. | Real | Africa |  |
| 1780 | OFICIAL | Spanish or Portuguese language countries [Official]. | Real | Europe | |
| 1781 | OH-MY-HAR | Fantasy by American Journal of Philately. | Not Scott Listed | None | |
| 1782 | OHU POST | Estonia, airmails. | Real | Europe | |
| 1783 | OIL RIVERS | Niger Coast Protectorate (1892-3). | Real | Africa | |
| 1784 | OKCA | Russia- Army of the North (1919) [Odjeiny Korpus SevernoyArmii]. | Real | Asia |  |
| 1785 | OLIVOYLA | Island Queendom of Popeye's girlfriend Olive Oyl. | Not Scott Listed | None | |
| 1786 | OLTRE GIUBA (overprint on stamps of Italy) | Oltre Giuba. | Real | Africa |  |
| 1787 | OLUJA | Croatia Overprint. | Not Scott Listed | Europe | |
| 1788 | OLYM | Greece. | Real | Europe | |
| 1789 | OLYMП | Greece. | Real | Europe | |
| 1790 | OLYMПIAKOI | Greece. | Real | Europe | |
| 1791 | OMAN, STATE OF | Bogus stamps for Arabian sultanate. Not to be confused with genuine Sultanate of Oman. | Not Scott Listed | Asia |  |
| 1792 | ON C.G.S. (overprint on stamps of India- Cochin) | India- Cochin, officials [On Cochin Government Service]. | Real | Asia |  |
| 1793 | ON K.D.S. (overprint) | India- Kishangarah, officials [On Kishengarh Durbar Service]. | Real | Asia | |
| 1794 | ON L.F.S. (overprint) | India, private overprints to prevent theft [On Local Funds Service]. | Real | Asia | |
| 1795 | ON S.S. (overprint) | India- Travanacore, officials. | Real | Asia | |
| 1796 | ON S.S.S. (overprint) | India- Sirmoor, officials [On Sirmoor State Service]. | Real | Asia | |
| 1797 | ON STATE SERVICE (overprint) | Iraq, officials. | Real | Asia |  |
| 1798 | ONZA | Spain, officials [Ounce]. | Real | Europe |  |
| 1799 | OPERATION PAPUA MERDEKA | Papua Freedom Movement. | Not Scott Listed | Australia |  |
| 1800 | ORANGE RIVER COLONY (overprint on stamps of Cape of Good Hope) | Orange River Colony (1900-2). | Real | Africa |  |
| 1801 | ORANJE VRIJ STAAT | Orange River Colony [Orange Free State]. | Real | Africa |  |
| 1802 | ORCHA | India- Orchha (1913-7). | Real | Asia | |
| 1803 | ORCHHA | India- Orchha (1913-7). | Real | Asia |  |
| 1804 | ORDINARY (overprint) | Liberia, airmail to regular mail conversion. | Real | Africa |  |
| 1805 | ORE (currency) | Denmark, Sweden or Norway. | Real | Europe |  |
| 1806 | ORGANISATION INTERNACIONALE POUR LES REFUGIES (overprint) | Switzerland- Int. Org. for Refugees, officials (1950). | Real | Europe |  |
| 1807 | ORGANISATION MONDIALE DE LA SANTE (overprint) | Switzerland- World Health Organization, officials. | Real | Europe |  |
| 1808 | ORO PASTAS | Lithuania, air mails. | Real | Europe |  |
| 1809 | ORPHELINS DE LA GUERRE | France, semipostals [War Orphans]. | Real | Europe |  |
| 1810 | ORTS-POST | Switzerland (1850). | Real | Europe | |
| 1811 | OSSETIA | Bogus Russian local overprint. | Not Scott Listed | Asia | |
| 1812 | OSTEN (overprint on stamps of Germany) | Poland- German Occupation (1939). | Real | Europe | |
| 1813 | OSTERR-POST | Austria. | Real | Europe | |
| 1814 | OSTERREICH | Austria. | Real | Europe | |
| 1815 | OSTERREICHISCHE POST | Austria. | Real | Europe | |
| 1816 | OSTLAND (overprint on stamps of Germany) | Russia- German occupation (1941-3). | Real | Europe |  |
| 1817 | OTVORENIE SLOVENSKENO (overprint on stamps of Czechoslovakia) | Slovakia (1939). | Real | Europe |  |
| 1818 | OUBANGUI-CHARI | Ubangi. | Real | Africa |  |
| 1819 | OUBANGUI-CHARI-TCHAD (overprint on stamps of Middle Congo) | Ubangi (1915-24). | Real | Africa |  |
| 1820 | OUDEYPOOR PALUMPOOR | Indian State produced as German fantasy issue in 1889. | Not Scott Listed | None | |
| 1821 | OДHA MAPKA YKSIMAPKKA | Finland. | Real | Europe | |
| 3798 | Obock | Obock (stamp issuing country) 1892 - 1894 | Real | Africa |  |
| 3799 | Oil Rivers Protectorate | Oil Rivers Protectorate (stamp issuing country) 1892 - 1893 | Real | Africa |  |
| 3800 | Oldenburg | Oldenburg (stamp issuing country) 1852 - 1867 | Real | Europe |  |
| 3801 | Oltre Juba | Oltre Juba (stamp issuing country) 1925 - 1926 | Real | Africa |  |
| 3802 | Oman | Oman (stamp issuing country) 1971 - | Real | Asia |  |
| 3803 | Orange Free State | Orange Free State (stamp issuing country) 1868 - 1900 | Real | Africa |  |
| 3806 | Orange River Colony | Orange River Colony (stamp issuing country) 1900 - 1907 | Real | Africa |  |
| 3807 | Orchha | Orchha (stamp issuing country) 1913 - 1939 | Real | Asia |  |
| 3808 | Ostland | Ostland (stamp issuing country) 1941 - 1945 | Real | Europe |  |
| 3809 | Ottoman Empire issues | Ottoman Empire issues (stamp issuing country) 1870 - 1913 | Real | Europe |  |
| 3810 | Oubangui Chari | Oubangui Chari (stamp issuing country) 1922 - 1937 | Real | Africa |  |
| 3811 | Oubangui Chari Tchad | Oubangui Chari Tchad (stamp issuing country) 1915 - 1922 | Real | Africa |  |