- orcus
- Oryzopsis miliacea
- one who knows the secret
- out of thin air
- old money
- outer canthus
- on the part of
- open_ended line
- one's own side
- on base
- on the right lines
- on the qt
- on the q.t.
- on request
- osteomalacia
- one's star to be in the ascendance
- one who adopts
- orthogenic evolution
- one's sleep to vanish
- oast
- one who studs or insets a gem (in an ornament etc.)
- one who lives on others' earnings
- open_handed
- out of touch
- orienting
- open necked
- of that kind
- outer mongolia
- opera cloak
- organic light_emitting diode
- oxalosuccinic acid
- organic farming
- on the run
- offer up
- old enough to know better
- out of all proportion to
- optical crown glass
- object routine
- one who can see through into the future
- oil future
- on the ropes
- one's own duty
- one who serves a drink (liquor)
- one iron
- ozarks
- ozark plateau
- Oncovin
- orangeade
- oropharyngeal
- orbicule