Author
- Oddo di Biagio1
- Odo of Meung1
- Oglerio di Lucedio2
- Okçuzade Mehmet Sahî1
- Oliver Maillard1
- Omar Khayyám5
- Onasander2
- Order of Saint Michael1
- Order of the Argonauts of Saint Nicholas1
- Order of the Garter1
- Order of the Immaculate Conception1
- Orderic Vitalis1
- Origen2
- Oronce Finé1
- Orosius1
- Osbern of Gloucester1
- Osman Süruri1
- Osman bin Keskin1
- Oswald Corda1
- Ottavio Strada1