| Acronym | Meaning |
 | OSERS | U.S. Department Of Education Office Of Special Education And Rehabilitative Services |
| OSETI | Optical SETI |
| OSI | Office Of Strategic Influence - Ontario Swine Improvement - Open Source Initiative - Open Systems Interconnection |
| OSIA | (U.S.) On-Site Inspection Agency |
| OSL | Open Source License - Optically Stimulated Luminescence - Orbiting Solar Laboratory |
| OSS | Office Of Strategic Services (1942?1945 |
| OSS | Ossetic Language (ISO 639-2 Code) |
| OTA | Office Of Technology Assessment -Orthodontic Technicians Association |
| OTB | Off The Ball -OneSAF Testbed (a Descendant Of ModSAF) |
| OTC | Over-The-Counter (drugs) |
| OTEA | (U.S. Army) Operational Test Evaluation Agency |
| OTI | Office Of Transition Initiatives (United States Agency For International Development) |
| OTO | Ordo Templi Orientis (Latin |
| OTOH | On The Other Hand |
| OTSBH | Over The Shoulder Boulder Holder |
| OVC | Overcast (METAR Code) |
| OVV | Optically Visually Violent |
| OW | Old Wykehamist |
| P | Peta - Phosphorus |
| P | Pico |
| P2P | Peer-to-peer (networking) |
| P3I OR P?I | PrePlanned Product Improvement |
| PA | Panama (ISO 3166 Digram) - Paraguay (FIPS 10-4 Country Code) -Partial Agonist -Pennsylvania (postal Symbol) - Petampere -PolyAmide (Nylon) - Prince Albert Piercing - Public Address (vocal Amplification System) - Public Affairs |
| PA | Pascal - Protactinium |
| PA | Picoampere |
| PA | Punjabi Language (ISO 639-1 Code) |
| PA 6 | Nylon 6 (PolyAmide) (similarly For PA 11 |
| PAA | Position Area For Artillery |
| PAB | Panamean Balboa (ISO 4217 Currency Code) |
| PABX | Private Automated Branch EXchange (company Phone System) |
| PAC | Pan Africanist Congress -Partial Solar Calibrator - Patriot Advanced Capabilities - Political Action Committee -Primary Auditory Cortex |
| PACOM | (U.S.) Pacific Command |
| PADI | Professional Association Of Diving Instructors |
| PAEK | PolyArylEtherKetone |
| PAFC | Plymouth Argyle Football Club |
| PAHO | Pan American Health Organization |
| PAI | PolyAmide Imide |
| PAK | Pakistan (ISO 3166 Trigram) |
| PAKISTAN | Punjab |
| PAL | Phase Alternating Line (TV Standard) -Philippine Airlines |
| PAM | Precision Attack Missile |
| PAMELA | Payload For Antimatter Matter Exploration And Light-nuclei Astrophysics |
| PAMELA | Pedestrian Accessibility And Movement Environment Laboratory |
| PAN | STARRS -Panoramic Survey Telescope And Rapid Response System |
| PAN | Panama (ISO 3166 Trigram) -Personal Area Network -PolyAcryloNitrile |
| PAN | Punjabi Language (ISO 639-2 Code) |
| PANS | Pretty Advanced/Awesome Network Services (cf. POTS) |
| PAO | Public Affairs Officer |
| PAPI | Precision Approach Path Indicator (aviation) |
| PARC | Palo Alto Research Center |
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