- otto von bismarck
- off hands
- off_axis reflector
- one who causes quarrels amongst people
- on foot
- on shanks mare
- on shanks pony
- onyxis
- or related with
- oogenium
- one who is made to work without remuneration
- one who or that which imparts knowledge
- one who or that which introduces
- Old Glory
- one who waits
- onboard
- omdurman
- omega_3
- ombudsman
- out for the count
- ornithological
- ornithology
- optical mark reading
- onco gene
- oncologic
- operator guidance code
- operator error
- off white
- open slide test
- open source
- one of the suits in playing cards—the spade
- ordinate
- order arms
- order paper
- old timer
- old tuberculin
- on that side
- our saviour
- or fit for keeping in
- optical aberration
- optical pit
- orthochromatic
- orthochromatic normoblast
- of vicious disposition
- optical binary search tree
- oximetry
- organic rigidity
- organic pain
- organic phosphorus
- order wire multiplex