- Old World crayfish
- on account of
- on the back side of
- okapia johnstoni
- okay
- okeh
- okey
- onwards
- orthodontic braces
- outside clinch
- off stage
- oldline
- old prussian
- Ouija
- offstage
- ofo
- outcome
- ovenbake
- ordination
- oscillatory
- oncology
- otology
- otolaryngology
- otorhinolaryngology
- ornery
- omnious
- overcasting
- overcast
- old bailey
- old as the hills
- opuntiales
- onion appearance
- onion roll
- offsides
- Old Church Slavic
- Old Bulgarian
- Old Church Slavonic
- optical condenser
- omental bursa
- on the one hand
- Oncidium papilio kramerianum
- Oncidium papilio
- orchis papilionaceae
- orange milkweed
- organization expense
- outspan
- outstay
- ooze through
- open weave
- outler macroinstruction