- one's word
- oesophagoscope
- oligodynamic activity
- opted out
- one half (of anything)
- open out
- on_line terminal
- on_line searching
- on_line terminal test
- on_line system
- on_line test system
- on_line input
- on_line testing
- on_line unit
- overflowing
- of fixed duration
- on and on
- open market
- open letter
- one's boat to sink
- outer
- only if
- occipital nerve
- outwards
- outwardly
- of ill_breed
- orang utan
- orang outang
- on the quiet
- one's bark to be worse than one's bite
- ocular movement
- ophthalmic injection
- over a barrel
- oil immersion lens
- of previous times
- open mindedness
- oscillate
- outwitted
- outdone
- opioidagonist
- of a proper type
- outscore
- outlive
- of a kind
- original writing
- old folks home
- old peoples home
- on the increase
- one behind the other
- otherness