List of nicknames of European royalty and nobility: O
- Oberto, Margrave of Massa and Corsica Brattoporrata, Brotoporrada 1
- Obizzo, Margrave, c1108 Malnitpore 2
- Ocko I of East Frisia the Elder
- Ocko II, Count of East Frisia the Younger
- Odet de Coligny Cardinal de Chatillon, the Protestant Cardinal 34 5
- Odette de Champdivers la Petite Reine
- Odo II of Blois le Champenois 6
- Odo, Count of Paris Capet
- Odo, Count of Valois (d.c.1085) the Mad
- Odo of Apulia the Good Marquis
- Odo of Aquitaine the Great
- Oddone (d.1104) Altasapada 7
- Olaf I of Denmark le Famelique, Olaf Hunger
- Olaf I of Norway the Sleeper, the Sleeping Hero, the Sleeping King
- Olaf I of the Isle of Man Morsel, the Dwarf
- Olaf II of the Isle of Man the Black 8
- Olaf Cuaran Hairy Brogues, Sandals 9
- Olaf Haraldsson Geirstadalf Olaf II, King of Vestfold and Jutland, the Elf of Geirstad (Nor. Geristad-alf, Geirstada-Alf, Geirstadaalf)
- Olaf Ingjaldsson, King of Dublin (835–871) the White
- Olaf of Denmark the Mild
- Olav I, King of the Isle of Man, 1097–1153 the Red
- Olav II of Norway the Big One, Digera, the Chapman, the Fat, the Saint, the Seaman
- Olav III of Norway the Mild, the Pacific 10
- Olav IV of Norway Harefoot
- Olav V of Norway the People's King
- Oleg I, Prince of Novgorod-Severskiy, 1097–1115 the Infamous
- Oleg I, Prince of Riazan, 1252–1258 Krasniy, the Fair, the Handsome 11
- Oleg II, Prince of Riazan, Grand Prince of Riazan, 1389–1402 the Great
- Oleg of Novgorod the Knowing, the Seer, the Wise
- Oleg Svyatoslavich, Prince Chernigov Gorislavich 12 1314 15
- Olga of Kiev Preskasa, the Beauty, the Saint 16 17
- Oliver Cromwell Crum-Hell, Ironsides , Nosey, Old Ironsides, Old Noll, Ruby Node, the Almighty Nose, the Archangel Who did Battle with the Devil, the Lord Protector of the Commonwealth 18 19
- Olivier de Clisson the Butcher
- Oliver St John the Dark Lanthorn of the Commonwealth20
- Olof of Sweden the Lap King , the Tax King, the Treasurer'' 21
- Olof Trätälja the Tree-Cutter, the Tree-Hewer, the Tree-Feller, the Wood-Cutter 22
- Olof the Sharp-sighted
- Ordoño IV of Leon the Bad, the Hunchback
- Orhan I the Fighter, the Hero of the Religion, the Leader, the Ruler, the Warrior of the Faith 23
- Orielda of Tirel the Noble Lady, wife of Fulk de Tirel, son of Ralph, Lord of Tirel and Poix
- Orlando of Fidenza the Magnificent
- Orlando Pallavicini (c1393-1457) the Magnificent 24
- Oshin of Armenia the Regent
- Osman I Little Osman, the Black, the Dark, the Fighter, the Founder, the Holy Warrior, the Pride of the Religion 25 26 27 28
- Osman I, King in Bithynia, 1282–1326 the Warrior of the Faith
- Osman II Farisi, the Martyr, the Younger, the Young Osman 29 30
- Oswald of Northumbria the Saint
- Oswine of Deira the Saint
- Ottar of Sweden Wend-crow
- Ottaviano degli Ubaldini the Cardinal 31
- Otto I of Bohemia (d.1091), Duke of Olmutz the Handsome
- Otto I, Count of Guelders (1150–1207) the Great
- Otto I, Duke of Brunswick-Göttingen, 1367–1394 the Evil
- Otto I, Duke of Brunswick-Luneburg (1204–1252) the Child
- Otto I, Duke of Brunswick-Luneburg, 1434–1445 the Lame
- Otto I, Duke of Saxony the Magnificent
- Otto I, Holy Roman Emperor the Great
- Otto II, Count of Habsburg
- Otto II (VI), Duke of Brunswick-Göttingen, 1367–1394 the Bad
- Otto II, Duke of Brunswick-Luneburg the Generous, the Liberal, the Magnanimous, the Victorious
- Otto II, Duke of Luneburg, 1464–1471 the Magnificent
- Otto II of Guelders Horsefoot 32
- Otto II, Holy Roman Emperor the Bloody, the Red (Rufus) 33
- Otto II, Margrave of Brandenburg (1147–1205) the Spendthrift, the Extravagant
- Otto II, Marquess of Montferrat, 1372–1378 Secondotto
- Otto II Wittelsbach, Duke of Bavaria the August, the Noble
- Otto II, Count of Zutphen, 1063–1113 the Rich
- Otto II, Count of Guelders and Zutphen, 1229–1271 le Paralyse
- Otto II, Margrave of Meissen, 1156–1190 the Rich
- Otto II, Count of Guelders the Lame 34
- Otto III (VII) of Brunswick-Gottingen Cocles, the One-Eyed
- Otto of Ballenstadt, Duke of Saxony, 1111–1123 the Rich
- Otto III of Weimar and Orlamunde (1100–1123) the Rich
- Otto III, Count of Zutphen, 1101–1113 the Rich
- Otto III, Duke of Brunswick-Göttingen, 1318–1344 the Mild
- Otto III of Orlamunde the Mighty
- Otto III, Duke of Swabia Otto of Schweinfurt, the White
- Otto III, Margrave of Brandenburg, 1220–1267 the Pious
- Otto III, Holy Roman Emperor the Infant, the Wonder of the World, the World's Wonder, Stupor Mundi 35
- Otto IV of Brandenburg Otto with the Arrow, Sagittarius
- Otto IV, Holy Roman Emperor Otto of Brunswick, the Proud
- Otto IV of Kirchberg the Great
- Otto IV, Duke of Bavaria der Abbacher
- Otto V, Duke of Bavaria, the Bavarian, the Lazy, the Pimple
- Otto V, Margrave of Brandenburg, 1267–1299 the Tall
- Otto VI, Count of Orlamunde (1271–1318) the Rich
- Otto VI, Margrave of Brandenburg, 1267–1286 the Short, the Little, "Small"
- Otto the Elder of Hesse the Elder
- Otto of Austria the Well-Beloved, the Merry
- Otto, Duke of Brunswick-Göttingen, 1394–1463 the Blind
- Otto of Austria the Wise
- Otto of Bamberg the Father of Monks 36, the Saint 37
- Otto of Nordheim Otto of Northeim
- Otto, Landgrave of Hesse the Younger
- Otto von Bismarck the Founding Father, the Man of Blood and Iron , the Iron Chancellor 38
- Otto-Henry, Duke of Burgundy the Great
- Ottokar I of Bohemia the Victorious
- Ottokar II of Bohemia the Great, the Iron King, the Iron and Gold King, the King of Gold and Iron 39
- Owain Goch ap Gruffydd the Red
- Owain Danwyn the White Tooth
- Owain ap Thomas, Prince of Gwynedd the Red Hand 40
- Owain Lawgoch Owain of the Red Hand
