- opium taker
- on top of
- oregon pine
- oriental spruce
- oriental scops owl
- old world least weasel
- opossum rat
- oporto
- Osage
- Omaha
- osteogenic sarcoma
- otto frisch
- otto robert frisch
- oppositive
- oval face
- os sesamoideum
- olga korbut
- ostracod
- ogden
- Ottawa
- Ogalala
- Onondaga
- Oneida
- olmsted
- orange toast
- onion butter
- oil geologist
- ortolan bunting
- oxanthrone
- orumiyeh
- opportunely
- one_upmanship
- ornamentalism
- ondaatje
- oviparvous
- okapi
- onychophora
- orcinus orca
- oxadiaza
- ordinary level
- ordinary bicycle
- out of the ordinary
- out of all proportion
- off the hook
- Ollari
- of the breadth of a nail —of very small breadth
- orthodox sleep
- off one's head
- off focus radiation
- old country