- outlet box
- official description
- outguess
- outbrave
- olfactory organ
- ombu
- oliguria
- organisational information system
- open up a can of worms
- overpopulate
- overlordship
- oriel window
- oriel
- omasum
- orange hawkweed
- optimum working frequency
- opposite word
- oxytone
- oncolytic drug
- oncorhynchus
- opening snap
- opening move
- openhearted
- opening (of eyes)
- onsager
- opening hours
- originate in
- offsaddle
- offset lithography
- offset printing
- output signal
- outsell
- outshine
- ordinal number
- ordinary seaman
- operating capital
- ordinary symbol
- OTC market
- outlander
- oldie
- opera house
- office building
- opium den
- ousting
- often
- ord kangaroo rat
- overspread
- ormuzd
- odalisque
- outbalance