- os palatinum
- optical pyrometer
- omicron
- Omani
- Ommiad
- Omayyad
- old line state
- old witch grass
- one_quadrillionth
- os trapezoideum
- old_time
- olde worlde
- old_fashionedness
- open_class word
- ordinariness
- orientalism
- one_quarter
- offensive
- oldwench
- oldwife
- oil_hardened steel
- oatcake
- one_quintillionth
- Old World rabbit
- Oryctolagus cuniculus
- omnidirectional antenna
- oil nut
- Old World mistletoe
- operational cell
- oftenness
- optic radiation
- order radiolaria
- osmerus mordax
- orb_weaver
- Orchestiidae
- Oniscidae
- outgrowth
- oxtail
- orthotomus sutorius
- Old World warbler
- onionskin
- open end fund
- other
- Oscan
- opv
- oral poliovirus vaccine
- oil lamp
- olav ii
- olaf ii
- Ostyak