Obwalden
Odessa, Republic of
Öland
Ogaden
Ohio
OIC - Islamic Conference
Oil Rivers Protectorate
Okinawa
Oklahoma
Oldenburg
Oman
Ontario
Opole (voivodeship of)
Orange River Colony
Orange Free State
Order of Alcántara
Order of Avis
Order of Calatrava
Order of Christ
Order of Dobrin
Order of Hospitallers
Order of Malta
Order of Montesa
Order of Montjoie
Order of St. Constantine
Order of St. John
Order of St. Lazarus
Order of Santiago
Order of the Sword Brothers
Order of the Holy Grave
Order of the Templars
Order of the Wing of St. Michael
Oregon
Organization of African Unity - OAU
Organization of American States - OAS
Organization of Central American States
Orkney Islands
Orléanais
Ottoman Empire
Ostyako-Vogulia
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