Encyclopedias, Almanacs, Transcripts, and Maps
- Old World Deer (Cervinae)
- Old World Flycatchers (Muscicapidae)
- Old World Fruit Bats I (Pteropus)
- Old World Fruit Bats II (All Other Genera)
- Old World Leaf-Nosed Bats (Hipposideridae)
- Old World Monkeys I (Colobinae)
- Old World Monkeys II (Cercopithecinae)
- Old World Orioles and Figbirds (Oriolidae)
- Old World Porcupines (Hystricidae)
- Old World Sucker-Footed Bats (Myzopodidae)
- Old World Warblers (Sylviidae)
- Old-Growth Forest
- Oldenberg, Hermann
- Oligochaeta (Earthworms)
- Olmec Religion
- Olms and Mudpuppies (Proteidae)
- Olmsted, Frederick Law, Sr. (1822 – 1903) American Landscape Designer
- Omayyads
- Omophagia
- on-line publishing