- OA
- Operability Additive
- OBD
- On-Board Diagnostics
- OCP
- Olefin Co-Polymer
- OECD
- Organisation for Economic Co-operation and Development
- OEM
- Original Equipment Manufacturer
- OICA
- Organisation Internationale des Constructeurs d’Automobiles (formerly BPICA)
- OIT
- Oxidation Induction Time
- OPEC
- Organization of the Petroleum Exporting Countries
- OPEST
- Oil Protection and Emission System Test
- ORI
- Octane Requirement Increase
