| Abbreviation | Definition |
|---|---|
| OF | Operation Flood |
| OFA | Orthopedic Foundation for Animals |
| OIE | Office International des Epizooties |
| OIE | World Organisation for Animal Health |
| OK | Orange Kitten |
| OK | Ornary Kitten |
| OLIVER | Online Library of Images for Veterinary Education and Research |
| OM | Orange Monkey |
| OMAFRA | Ontario Ministry of Agriculture, Food and Rural Affairs |
| OMSCO | Organic Milk Suppliers Cooperative |
| OMVS | Oro Medonte Veterinary Services |
| OP | Olfactory Pit |
| OPU | Ovum Pick-Up |
| ORPRC | Oregon Regional Primate Research Center |
| ORT | Ornithobacterium RhinoTracheale |
| OS | Orthopterists' Society |
| OSERA | Ohio Scientific Education & Research Association |
| OV | Orkney Vole |
| OVC | Ontario Veterinary College |
| OVERT | Oregon Veterinary Emergency Response Team |