Encyclopedias, Almanacs, Transcripts, and Maps
- Oak Tree
- Oak-Apples
- Oakley G. Kelly
- Oaks
- Oaks
- Oaten, Ernest W(alter) (ca. 1937)
- Oath of Asaph
- Oath of Hippocrates
- Oath of Initiation (Caraka Samhita)
- Oath of Soviet Physicians
- Obercit, Jacques Hermann (1725-1798)
- Oberion
- Object Reading
- Objective Phenomena
- Obligation and Supererogation
- Observing and Defining Comets
- Obsession and Possession
- Obstetrics in the 1700s
- Occidental Society of Metempiric Analysis
- Occipital Neuralgia