- order of seniority
- of sound mind
- ophthalmoscopic topography
- ophthalmosynchysis
- osseous spiral lamina
- one over the other
- of own accord
- orbitale
- orbito sphenoid
- on acquaintance
- on air
- on the way out
- obturator internus
- obturator lymph node
- obturator externus
- one who asks
- one who looks after
- one who affords a protection
- oil of neroli
- oriflamme
- ojos del salado
- one gets an opportunity only once in a while
- on the never never
- on the air
- o'casey
- o'hara
- or somebody
- on behalf of
- one who subsists on milk alone
- office information system
- oncotherapy
- on high
- or operation
- orcein
- one whose achievements are
- original language
- optical method
- one who mentions or discusses
- opisthogathous
- one who misdirects or propagates ignorance through one's writings
- one who wields a
- one who takes (e.g.
- one whose profession is loading
- one who recites the
- one who sleeps on the ground
- one who proves
- one who rides a chariot
- orbital motion
- orbital rotation
- ourselves