- O'Daly, Carroll (Cearbhall Ó Dálaigh)
- O'Kelly, Sean Thomas (Seán Tomás Ó Ceallaigh)
- Obama, Barack
- Obregón, Álvaro
- Oman: Archontology
- Oman: Heads of State: 1749-2024
- Oman: Notes
- Oman: Polity Style: 1749-2024
- Oman: Sources
- Orange Free State: Archontology
- Orange Free State: Heads of State: 1854-1902
- Orange Free State: Polity Style: 1854-1902
- Orange Free State: Sources
- Order of Malta: Archontology
- Order of Malta: Grand Masters: 1530-1798
- Order of Malta: Grand Masters: 1798-1803
- Order of Malta: Grand Masters: 1803-2024
- Order of Malta: Official Style: 1530-2024
- Order of Malta: Sources
- ORLÉANS, Louis-Philippe d'
- Ortiz Rubio, Pascual
- Osorio de Moscoso, Vicente Isabel (Marqués de Astorga)
- Ottoman Empire: Archontology
- Ottoman Empire: Notes
- Ottoman Empire: Polity Style: 1453-1922
- Ottoman Empire: Rulers: 1453-1922
- Ottoman Empire: Sources
Last update: 21 Nov 2024