- osmerus eperlanus
- osmilloscope
- onion salt
- overmantel
- one who gives
- one who owes
- one who wants to repay (a loan etc.)
- oracle of apollo
- oracle of delphi
- off limits
- old english sheepdog
- orthotonous
- old world white pelican
- oppressive and laborious (business)
- oppressive
- oppressive rule
- ornithomimid
- one_piece
- open eyes to
- open drop method
- open end investment company
- othello
- other worldly
- other woman
- other world
- of or pertaining to a school
- oscan_speaking
- osborne
- open_hearth process
- oral poliovaccine
- oil rigger
- one who or that which spells devastation
- open marriage
- offerings made to an idol (and later distributed amongst the devotees)
- one whose very sight is as good as a visit to a holy place of pilgrimage
- omnipotent
- Otaheite arrowroot
- one gifted with a vision to see through the past
- oil level gauge
- Oncorhynchus kisutch
- Oncorhynchus nerka
- otoscope
- ophthalmoscope
- organon
- ostyak_samoyed
- order Volvocales
- order Heterotrichales
- order Chlorococcales
- order Charales
- order Laminariales