Encyclopedias, Almanacs, Transcripts, and Maps
- Onganía, Juan Carlos (1914–1995)
- Ontological Argument for the Existence of God
- Ontology, History of
- Ontong Java
- Opata
- Operation Bootstrap
- Operationalism
- Oppida
- Orang Asli
- Oraon
- Oraons
- Orbach, Jerome Bernard (“Jerry”)
- Orbegoso, Luis José de (1795–1847)
- Orbigny, Alcide Dessalines d' (1802–1857)
- Orbison, Roy Kelton
- Orbón, Julián (1925–1991)
- Orcadians
- Ordenações do Reino
- Ordenanza de Intendentes
- Ordinances on Veiling