OA
- OA – (i) Operational Analysis – (i) Overeaters Anonymous
- OAC – (i) On Approved Credit
- OAP – Old Age Pensioner (UK)
- OAS – (i) Organization of American States
- OARSI – (i) Osteoarthritis Research Society International
- OASIS – (a) Objects and Agents for Information Systems and Simulation – Organization for the Advancement of Structured Information Standards
- OASIS – (a) Ontario Agencies Supporting Individuals with Special Needs
- OASIS – (a) Ontario Association of Sewage Industry Services
- OASP – (i) Operational Analysis Support Paper/Plan
- OAU – (i) Organization for African Unity
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