Encyclopedias, Almanacs, Transcripts, and Maps
- Omega Directory
- Omez, Réginald, O.P. (Order of Dominicans) (1895-?)
- Omphalocele
- Omphalocele
- Omphalomancy
- Oncogene Research
- One-to-One Correspondence
- Onec, Omnec
- Oneiromancy
- One–to–one Correspondence
- Onimancy (or Onycomancy)
- Onomancy (or Onomamancy)
- Onychomancy
- Oom the Omnipotent
- Oomancy
- Ooscopy and Oomantia
- Opahs and Relatives: Lampridiformes
- Open Deck
- Open Mind Magazine
- Open Society