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• O'Daly, Carroll (Cearbhall Ó Dálaigh)
• O'Kelly, Sean Thomas (Seán Tomás Ó Ceallaigh)
• Obama, Barack
• Obregón, Álvaro
• Oman: Archontology
• Oman: Heads of State: 1749-2019
• Oman: Notes
• Oman: Polity Style: 1749-2019
• 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: Heads of the Order: 1530-1798
• Order of Malta: Heads of the Order: 1798-1803
• Order of Malta: Heads of the Order: 1803-2019
• Order of Malta: Official Style: 1530-2019
• 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

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