Oroszlánkő Castle

Hungary

Ortegat Castle

Belgium

Ortnek Castle

Slovenia

Orăștie

Romania

Oršić Castle

Croatia

Oršić-Slavetić Castle

Croatia

Osaka Castle

Japan

Osbyholm Castle

Sweden

Private residence

Oscarshall palace

Norway

Osgood Castle

the United States

Oshi Castle

Japan

Osiek

Poland

Oslo Old Bishop's Palace

Norway

Ruin

Oslo ladegård

Norway

Osowiec Fortress

Poland

Ossolin

Poland

Ostrogski Castle

Poland

Ostroh Castle

Ukraine

Ostroróg Castle

Poland

Ostrovica Castle

Bosnia and Herzegovina

Ostrožac Castle

Bosnia and Herzegovina

Ostrvica

Serbia

Ruins

Ostrzeszów

Poland

Ostróda

Poland

Ostrý Kameň Castle

Slovakia

Osu Castle

Ghana

Otaki Castle

Japan

Otepää Castle

Estonia

Othello CastleΚάστρο της Oθέλλου / Othello Kulesi

Cyprus

14th century

Otmuchów Castle

Poland

Otočac Fortress

Croatia

Ottegraven Castle

Belgium

Otyń

Poland

Ouborg Castle

the Netherlands

Oulu Castle

Finland

Totally ruined

Ouranoupolis, Chalkidiki

Greece