| O2 | Oxygen |
| OASIS | Organization for the Advancement of Structured Information Standards |
| OBD | On-Board Diagnostic |
| OBD-II | On-Board Diagnostic II |
| OCP | Olefin Copolymer |
| ODI | Oil Drain Interval |
| ODO | Odometer |
| OE | Original Equipment |
| OEM | Original Equipment Manufacturer |
| OHC | Overhead Camshaft |
| OHV | Overhead Valve |
| OICA | Organisation Internationale des Constructeurs d’Automobiles (formerly BPICA) |
| OIT | Oxidation Induction Time |
| OLED | Organic Light Emitting Diode |
| OPEC | Organization of Petroleum Exporting Countries |
| OPEST | Oil Protection and Emission System Test |
| ORD | Octane Requirement Decrease |
| ORI | Octane Requirement Increase |
| OSHA | Occupational Safety and Health Administration |
| OTAG | Ozone Transport Assessment Group |
| OTAQ | Office of Transportation and Air Quality (US EPA) |
| OTC | Overhead Trip Computer |
| OTIS | Overhead Travel Information System |
| OXCAT | OXidation CATalyst |
