CIO
- CIO (magazine)
- Central Imagery Office
- Central Intelligence Office
- Central Intelligence Organisation
- Centro de Investigaciones en Optica
- Chairperson-in-office
- Charitable incorporated organisation
- Chief information officer
- Chief innovation officer
- Chief investment officer
- City One station
- Comité international olympique
- Commonwealth Chairperson-in-Office
- Congress of Industrial Organizations
- Conventional International Origin
- Corruption Investigation Office for High-ranking Officials
- Credit and Investments Ombudsman
- Ferber method
- JFS (file system)