- old growth
- old codger
- old frisian
- old girl
- old maidish
- of world fame
- orange peel fungus
- optical telescope
- orphenadrine
- orange liqueur
- Ortilis vetula macalli
- open calumny
- orpine
- orr
- Ophisaurus
- Opheodrys
- Ophiophagus
- outdoor man
- ordinal trait
- organ_specific auto_antibody
- Ostwald's theory of indicators
- organicism
- Old World jay
- organized labor
- olfactory bulb
- Orion
- ornamental trees
- osteology
- olive_like
- oliver
- olive brown
- olive_grey
- olive_coloured
- olivary fibre
- olea europaea
- outer hebrides
- oilfield
- oil tanker
- oilbird
- orthoclase
- oligoclase
- oncorhynchus tshawytscha
- old boy network
- oil business
- oil industry
- onychophoran
- omentum
- old world scops owl
- overeating
- open sight