- O'Brien, James (Bronterre)
- O'Brien, John
- O'Brien, Michael
- O'Brien, Richard Barry
- O'Brien, William Smith
- O'Brien-Stafford → Stafford, Augustus
- O'Carolan → Carolan, Turlough
- O'Clery, Michael
- O'Connel → O'Connell, Charles Underwood
- O'Connell, Charles Underwood
- O'Connell, Daniel
- O'Connell, Martin
- O'Connor, Arthur
- O'Connor, Feargus Edward
- O'Connor, James
- O'Connor, Roger
- O'Conor, Charles
- O'Conor, Charles
- O'Conor, Matthew
- O'Curry, Eugene
- O'Daly → Daly, James D.
- O'Daly, Demetrio
- O'Doherty, Cahir
- O'Donnell → McDonnell, Robert
- O'Donnell, Alejandro
- O'Donnell, Carlos
- O'Donnell, John
- O'Donnell, José
- O'Donnell, Joseph
- O'Donnell, 1st Earl of Tyrconnell, Rory
- O'Donnell, conde de La Bisbal, Enrique José
- O'Donnell, duque de Tetuán, vizconde de Aliaga, conde de Lucena, Leopoldo
- O'Donoghue, Daniel
- O'Donojú, Tomás
- O'Donovan, John
- O'Donovan Rossa, Jeremiah
- O'Donovan Rossa, Mary Jane (Molly)
- O'Farrill, Gonzalo
- O'Flaherty, Edmund
- O'Halloran
- O'Leary
- O'Leary, Ellen
- O'Leary, John
- O'Mahony, Thaddeus
- O'Meagher Condon, Edward
- O'Meara, Barry Edward
- O'Neil, Arthur George
- O'Neill of Kinard, Phelim
- O'Neill, 2nd Earl of Tyrone, Hugh
- O'Shea, Enrique
- O'Shea, Henry
- O'Sullivan, James
- Oakman
- Oastler, Richard
- Oberin des Klosters "Schwestern der Vorsehung"
- Obermüller, Wilhelm
- Oberwinder, Heinrich
- Oberwinter → Oberwinder, Heinrich
- Obolenskaya, Zoya Sergeevna
- Obolenski → Obolenskaya, Zoya Sergeevna
- Oborski, Ludwik
- Obrenovič (Обреновић), Miloš
- Obreskov, Aleksej Michajlovič
- Obručev (Обручевъ), Nikolaj Nikolaevič
- Ocho, el
- Ochsenbein, Ulrich
- Ochterlony, David
- Ockenfuß → Oken, Lorenz
- Octavia
- Octavius, Gaius
- Odell, Joseph
- Odermann, Karl Gustav
- Odger, George
- Odgers → Odger, George
- Odhin → Odin
- Odin
- Odoardo I. Farnesio
- Odoardo II. Farnesio
- Odoario
- Odysseus
- Oecolampadius, Johannes
- Oelbermann, Hugo
- Oelze, F.
- Oerindur
- Oettinger, Eduard Maria
- Ofenheim, Ritter von Ponteuxin, Victor
- Offenbach, Jacques
- Offers → Juta, Maria
- Ogareff → Ogarev , Nikolaj Platonovič
- Ogarev, Nikolaj Platonovič
- Ogarjow → Ogarev , Nikolaj Platonovič
- Ogé, Julie
- Ogilvy, Gabriel
- Oginski, Michal Kleofas
- Ohage, Georg
- Ohm
- Ohm, Georg Simon
- Ohnstein, Sigismund
- Ohsson, Ignace Mouradgea d’
- Oidipus
- Oisin → Ossian
- Okeanos
- Oken, Lorenz
- Okolowicz, Auguste Adolphe
- Ol'ga
- Ol'ga Nikolaevna (Ольга Николаевна)
- Olaf → Amlaíb Cuarán
- Olavide, Pablo Antonio de
- Olberg, Friedrich Wilhelm Eduard von
- Old Becker → Becker, Johann Philipp
- Old Nick, Nickey, Nicky → Marx, Karl
- Oldenberg
- Oldenburg
- Oldham, William Simpson
- Oldradus de Ponte
- Oldrini, Alessandro
- Olea, Alonso de
- Olenschlager, Johann Daniel von
- Oliphant, Laurence
- Oliveira Martins, Joaquim Pedro de
- Oliver, Samuel
- Oliver Twist
- Oliveros, Antonio
- Olivier → Ollivier, Émile
- Olivier, 1st Baron, Sydney Haldane
- Ollech, Karl Rudolf von
- Ollendorff, Heinrich
- Ollini, Giovanni Paolo
- Ollivier, John
- Ollivier, Jules
- Ollivier, Émile
- Olmsted, Frederick Law
- Olshausen, Theodor
- Oltra
- Oluf
- Olózaga, Salustiano de
- Omar I., ibn al-Chattab
- Omdat-ul-Omrah → Umdat ul-Umara
- Omer-Sheik-Mirsa → Umar (Omar) Shaikh Mirza II
- Ompteda, Dietrich Heinrich Ludwig von
- Omst
- Oncken, Hermann
- Onegin
- Oniegin → Onegin
- Onís, Mauricio Carlos de
- Opdyke, George
- Opitz, Theodor
- Oppas
- Oppenheim → Herz
- Oppenheim → Kugelmann, Gertrud
- Oppenheim
- Oppenheim, Abraham (seit 1868) von
- Oppenheim, Heinrich Bernhard
- Oppenheim, Henri
- Oppenheim, Henriette
- Oppenheim, Isaak
- Oppenheim, Maximilian
- Oppenheim, Simon (seit 1867) von
- Oppenheim, Simon Lazarus
- Oppenheimer → Kugelmann, Rebecca
- Oppler, A.
- Oratti
- Ord, William Miller
- Orestes, Flavius
- Orestes und Pylades
- Orges, Hermann
- Origenes
- Orisio Santo
- Orive, José
- Orléans
- Orléans, Elisabeth Charlotte, duchesse d’
- Orléans, Gaston, duc d’
- Orléans, Marie Amélie Françoise Hélène, princesse d’
- Orléans, Philippe Ier, duc d’
- Orléans, Philippe II, duc d’
- Orléans, comte de Paris, Louis Philippe Albert d’
- Orléans, duc de Chartres, Robert Philippe Louis Eugène Ferdinand d’
- Orléans, duc de Montpensier, Antoine Marie Philippe Louis d’
- Orléans, duc d’ Aumale, Henri Eugène Philippe Louis d’
- Orloff → Orlov (Орловъ), Vasilij Ivanovič
- Orlov, Nikolaj Alekseevič
- Orlov (Орлов), Aleksej Fedorovič
- Orlov (Орловъ), Vasilij Ivanovič
- Ormuzd
- Ornan
- Oropesa, Manuel Joaquín Álvarez de Toledo, conde de
- Orosz, Gyula
- Orozco, José María
- Oršanskij (Оршанскій), Il’ja Grigor’evič
- Orsi, Graf
- Orsini, Anna, princesa
- Orsini, Cesare, conte di
- Orsini, Felice, conte di
- Ortes, Giammaria
- Orth, Georg Heinrich
- Ortica → Ortiga
- Ortiga
- Ortiz de Cervantes, Juan
- Orton, Arthur
- Ory
- Oráa, Javier Antonio Marcelino de
- Osborne, John
- Osborne siehe Bernal Osborne
- Osborne, (seit 1674) Earl of Danby, (seit 1689) Marquess of Carmarthen, (seit 1694) 1st Duke of Leeds, Thomas
- Oscar → Oskar II. (Schweden),
- Oscar I. → Oskar I. (Schweden)
- Oseroff → Ozeroff, Vladimir Aleksandrovič (Woldemar)
- Oserow → Ozeroff, Vladimir Aleksandrovič (Woldemar)
- Osiander, Heinrich Friedrich
- Oskar II. (Schweden)
- Oskar I. (Schweden)
- Osman
- Osman Bei
- Osman Pascha
- Osman Pascha
- Osman III.
- Ospak → Óspak
- Ossat, Arnauld d’
- Ossian
- Ossorio, Francisco de Paula
- Osten-Saken (Остен-Сакен), Dmitrij Erofeevič
- Osterhaus, Peter Joseph
- Ostermann, Andrej Ivanovič
- Osterroth → Overbeck, Helena
- Osterroth, Addy
- Osterroth, Agnes
- Osterroth, Friedrich
- Osterroth, Johann Wilhelm
- Ostertag, Johannes
- Osti, Christine
- Ostolaza, Blas Gregorio de
- Ostrovskij, Aleksandr Nikolaevič
- Ostrowski, Antoni Jan
- Ostrowski, Józefat Bolesław
- Ostrowski, Tamasz Adam
- Ostwald, Wilhelm Friedrich
- Ostyn
- Osuna, Francisco de Borja Bruno Téllez Girón
- Osuna, Mariano Téllez Girón de
- Oswald, Caroline
- Oswald, Ernst
- Oswald, Eugen(e)
- Oswald, Lina
- Othello
- Othen → Dickmann, Hermann
- Othman → Osman
- Othon de Grasse
- Othon I.
- Othon V.
- Otrep'ev → Lžedmitrij I.
- Otter Cordatès → Otterbein, Frédéric (gen. Fritz)
- Otterbein, Charles
- Otterbein, Frédéric (gen. Fritz)
- Otterstaetter, Otto
- Otto → Bebel, Johanna Caroline Julie
- Otto → Otto-Reventlow, Karl
- Otto, Carl Wunibald
- Otto I. der Große
- Otto von der Weiden → Corvin-Wiersbitzki, Otto von
- Otto-Reventlow, Karl
- Otto-Walster, August
- Otway, 3rd Baronet, Sir Arthur John
- Oudet, Émile
- Oudinot, Nicolas Charles Victor
- Oudinot, duc de Reggio, Charles Nicolas
- Ouseley, Gore
- Outine → Utin, Nikolaj Isaakovič
- Outine → Utina, Natalja Jeronimowna
- Outram, 1st Baronet, Sir James
- Ouvrard, Gabriel-Julien
- Ovalle, Félix
- Overbeck → Engels, Wilhelmine Julie
- Overbeck, Heinrich
- Overbeck, Helena
- Overbeck, Johannes Adolf
- Overton, Richard
- Ovid → Ovidius Naso, Publius
- Ovidius Naso, Publius
- Owen, Robert
- Owen, Sir Richard
- Owen, Stephen
- Owen, William
- Owen Meredith → Bulwer-Lytton, 1st Earl of Lytton, Edward Robert
- Oxenstierna, Erik
- Oxenstierna von Södermöre, Axel Gustavson
- Oyance, N.
- Ozeroff, Vladimir Aleksandrovič (Woldemar)
- Ozerov → Ozeroff, Vladimir Aleksandrovič (Woldemar)
- Oßwald, Ernst
- Oñaz y Loyola → Ignatius von Loyola