Words by letter O
- Open co-ordination method
- Openness, see Transparency
- Operation Odysseus
- Opinion
- Opt-out
- Optimum Currency Area
- Organic Farming
- Organic law
- Organisation for Economic Co-operation and Development, OECD.
- Organisation for Security and Co-operation in Europe, OSCE
- Organised crime, see Justice and Home Affairs
- Organization for European Economic Coorperation, OEEC
- Origin
- Outermost regions
- Overseas Countries and Territories, OCTs