Words by letter O
- OLAF
- Olympic circles, see Variable geometry
- Ombudsman
- Open co-ordination method
- Openness, see Transparency
- Operation Odysseus
- Opinion
- Opt-out
- Optimum Currency Area
- Orange card - See Subsidiarity
- Ordinary legislative procedure
- Organic Farming
- Organic law
- Organised crime, see Justice and Home Affairs
- Organization for European Economic Coorperation, OEEC