| Name of Family or Major Holdings -O- | Place of Origin |
| Omelas [viscount of; Barcelona] | Iberia |
| d'Orbec [de Thumery] | France |
| von Orlamünde | Ascania* |
| Seigneur d'Orval | Albret* |
| Osona [count of; Barcelona] | Iberia |
| Oxenbridge of Burton, Viscount [Monson] | UK |
| Oxenstierna | Sweden |
| Oxenstierna af Croneborg [Oxenstierna] | Sweden |
| Oxenstierna af Eka och Lindö [Oxenstierna] | Sweden |
| Oxenstierna af Korsholm och Wasa [Oxenstierna] | Sweden |
| Oxenstierna af Södermöre [Oxenstierna] | Sweden |