Encyclopedias, Almanacs, Transcripts, and Maps
- Orgy: Orgy in Asia
- Orgy: Orgy in Medieval and Modern Europe
- Orgy: Orgy in the Ancient Mediterranean World
- Ormuzd
- orogenesis
- Orphic Gold Tablets
- Orr, David W. (1944 – ) American Environmental Writer
- Orthodoxy and Heterodoxy
- Orthonectida (Orthonectidans)
- orthopaedics
- Orthoptera (Grasshoppers, Crickets, and Katydids)
- ortolan
- Osage Religious Traditions
- Osborn, Henry Fairfield (1887 – 1969) American Conservationist and Naturalist
- oscillating Universe theory
- Osmeriformes (Smelts, Galaxiids, and Relatives)
- Osteoglossiformes (Bony Tongues and Relatives)
- Ostracoda (Mussel Shrimps)
- Ostriches (Struthionidae)
- Otherworld