| OAA | Old Age Assistance; Older Americans Act; Opticians Association of America; oxaloacetic acid |
|---|---|
| OAB | ABO blood group; old age benefits |
| OAS | old age security; oral allergy syndrome; osmotically active substance |
| OASDHI | Old Age, Survivors, Disability and Health Insurance |
| OASDI | Old Age Survivors and Disability Insurance |