| O'CLOCK | Of the Clock 
| Others |
| O'er | Over (As in, "O'er yonder lay the green fields of home.")
| Others |
| O+I | Outings and Innings
| Others |
| O+O | Offline Plus Online
| Others |
| O+O | Offline plus Online / O plus O
| Others |
| O+O | Online and Offline
| Others |
| O-AMP | Ontario Automotive Modernization Program
| Others |
| O-CONUS | Outside Continental United States
| Others |
| O-Day | Off-Load Day
| Others |
| O-EA | O-flex Energy Absorbing
| Others |
| O-M | Opsoclonus-Myoclonus
| Others |
| O-MaSE | Organization-based Multi-agent System Engineering
| Others |
| O-NAV | Onboard Navigation
| Others |
| O-T-C | Over-The-Counter
| Others |
| o. g. | oben genannt
| Others |
| o. Prof. | ordentlicher Professor
| Others |
| O.A. | Order of Australia
| Others |
| O.A.M. | Medal of the Order of Australia
| Others |
| O.C.G.A. | Official Code of Georgia Annotated
| Others |
| o.h.,qq.h. | Omni hora, Quaque hora Every Hour
| Others |
| o.h.,qq.h. | OMNIHORA,Quarahora Every hour
| Others |
| O.M.D./D.O.M | Oriental Medical Doctor / Doctor or Oriental Medicine
| Others |
| O.o | Confusion
| Others |
| O.O | Surprised. Confused. Shocked.
| Others |
| O.O.D. | Operations Officer of the Day
| Others |
| O.P. | Osmotic Pressure
| Others |
| O.P.D. | Once Per Day
| Others |
| O.R.N. | Operating Room Nurse
| Others |
| O.S. | left eye (ocularis/oculus sinister (Lat.)
| Others |
| o.s. | oculus sinister - left eye
| Others |
| O.T.R. | Occupational Therapist Registered
| Others |
| o.u. | oculus uterque - both eyes
| Others |
| O/A | Old Age
| Others |
| O/A | On Account
| Others |
| O/A | On Admission
| Others |
| O/A | On Arrival
| Others |
| O/A | Open Account
| Others |
| O/A | Operating As
| Others |
| O/B | On Board
| Others |
| O/B | OnBoard
| Others |
| O/B | Operational Base
| Others |
| O/B | Outboard
| Others |
| o/b | overboard
| Others |
| O/C | Observer/Collector
| Others |
| O/C | Observer/Controller
| Others |
| O/C | Operations Critical
| Others |
| O/D | On Dock
| Others |
| O/D | Overdrive
| Others |
| O/D PL | Old Dog Place
| Others |
| O/D POOL | OutDoor Pool
| Others |