| O&S | Operations and Support 
| Others |
| O.S. | left eye (ocularis/oculus sinister (Lat.)
| Others |
| o.s. | oculus sinister - left eye
| Others |
| O/S | Operations and Support life cycle phase
| Others |
| O/S | Opposite Side
| Others |
| O/S | Out of Stock
| Others |
| O/S | Out Standing
| Others |
| O/S | OverSeas
| Others |
| OS | A On Surveillance
| Others |
| OS | A Outside System
| Others |
| OS | Left eye (oculus sinister)
| Others |
| OS | mouth (os)
| Others |
| OS | O Sullivan
| Others |
| OS | Obese Strain
| Others |
| OS | Object Summary
| Others |
| OS | Observational Study
| Others |
| OS | Occulus Sinister
| Others |
| OS | Occupational Safety
| Others |
| OS | Oceanography Society
| Others |
| OS | Ocular Sinister
| Others |
| OS | Oculum Sinister
| Others |
| OS | Oculus Sinister
| Others |
| OS | Odd Squad
| Others |
| OS | Off Screen
| Others |
| OS | Off Site
| Others |
| OS | Off-Street parking
| Others |
| OS | Offentlig Sak
| Others |
| OS | Office Surgery
| Others |
| OS | Officer Standpoint
| Others |
| OS | OfficerScout
| Others |
| OS | Oft S
| Others |
| OS | Oh Shoot
| Others |
| OS | Oh Starcraft
| Others |
| OS | Old School
| Others |
| OS | Old Sebilis
| Others |
| OS | Old Shinola
| Others |
| OS | Old Skool
| Others |
| OS | Old Style
| Others |
| OS | Old Suisun
| Others |
| OS | Ole Syversen
| Others |
| OS | Oleum Sanctum
| Others |
| OS | oligosaccharide
| Others |
| OS | Omega Strain
| Others |
| OS | Omenn syndrome
| Others |
| OS | Omnipotent Spirituality
| Others |
| OS | OmniSky
| Others |
| OS | On Sea
| Others |
| OS | On Serotta
| Others |
| OS | On Service
| Others |
| OS | On Sheet
| Others |
| OS | On Station
| Others |
| OS | On Steroids
| Others |
| OS | One shot
| Others |
| OS | One Size
| Others |
| OS | Oneself
| Others |
| OS | Online Service
| Others |
| OS | Online Services
| Others |
| OS | Only So
| Others |
| OS | Oooh Shizzit
| Others |
| OS | Open Server
| Others |
| OS | Open Socket
| Others |
| OS | Open Sockets
| Others |
| OS | Open Software
| Others |
| OS | Open Source
| Others |
| OS | Open Space
| Others |
| OS | Open Street
| Others |
| OS | Open System
| Others |
| OS | Open Systems
| Others |
| OS | Opening Snap
| Others |
| OS | Operating Software
| Others |
| OS | Operating Standards
| Others |
| OS | Operating Strength
| Others |
| OS | Operating Style
| Others |
| OS | Operating System
| Others |
| OS | Operational Software
| Others |
| OS | Operational Sponge
| Others |
| OS | Operations Specialist
| Others |
| OS | Operations System
| Others |
| OS | Ophiological Services
| Others |
| OS | Opitz syndrome
| Others |
| OS | Opportunity to Serve
| Others |
| OS | Opposite Sign
| Others |
| OS | Opposite Strand
| Others |
| OS | Optical Stabilizer
| Others |
| OS | Optics Subsystem
| Others |
| OS | Optimal Strategy
| Others |
| OS | Optimize for Size
| Others |
| OS | Optional Stopping
| Others |
| OS | Oracle Support
| Others |
| OS | Oral Solution
| Others |
| OS | Oral Surgery
| Others |
| OS | Orbital Servicing
| Others |
| OS | Orbiter Specification
| Others |
| OS | Orchard Street
| Others |
| OS | Ordnance Survey
| Others |
| OS | Organ Series
| Others |
| OS | Organ-Specific
| Others |
| OS | Organizational Software
| Others |
| OS | Organizzazioni Sindacali
| Others |
| OS | Orientation Sessions
| Others |
| OS | Original Solution
| Others |
| OS | Original Special
| Others |
| OS | Orthopaedic Surgeon
| Others |
| OS | Orthopterists' Society
| Others |
| OS | Orzya Sativa
| Others |
| OS | Osgood-Schlatter
| Others |
| OS | Osmium
| Element |
| OS | Osteogenic Sarcoma
| Others |
| OS | Osteogenic Sarcoma (bone tumors)
| Others |
| OS | osteosarcoma
| Others |
| Os | Osteosclerosis
| Others |
| OS | OStium (mouth)
| Others |
| OS | Ostrich Source
| Others |
| OS | Other Singer
| Others |
| OS | Other Source
| Others |
| OS | Other Specifications
| Others |
| OS | Other Supplementary
| Others |
| OS | Out Sized
| Others |
| OS | Outer Segment
| Others |
| OS | Outer Sen
| Others |
| OS | Outer Sensor
| Others |
| OS | Outer Space
| Others |
| OS | Output Swing
| Others |
| OS | Outside Scope
| Others |
| OS | Outside Shot
| Others |
| OS | Outside Source
| Others |
| OS | Outstanding Student
| Others |
| OS | Over Sized
| Others |
| OS | Overall Survival
| Others |
| OS | Overhead Sending
| Others |
| OS | Overhead Spending
| Others |
| OS | Overlap Syndrome
| Others |
| OS | Overly Short
| Others |
| OS | Overseas Studies
| Others |
| OS | overstock(ed)
| Others |
| OS | Overtly Similar
| Others |
| OS | Ownage Squad
| Others |
| OS | Oxidative Stress
| Others |
| OS | Oxygen Saturation
| Others |