 Chief Information Officer | Business |
| Carrier Insertion Oscillator | Common |
| Check it Out | Chat and Slang |
| Cisco Information Online | Technology |
| Congress of Industrial Organizations | Common |
| Corporate Information Officer | Science |
| Customer Integration Office | Technology |
| Center Information Officer | Science |
| Central Imagery Office | Military |
| Central Input/Output | Common |
| Central Intelligence Organisation | Organizations |
| Chairman in Office | Military |
| Channel Input/Output | Technology |
| Chief Information Office | Science |
| Chief Intelligence Officer | Military |
| Chief Internet Officer | Common |
| Coal-in-Oil | Common |
| Collège d'Implantologie Orale | Medical |
| Command Issuing Office | Military |
| Common Item Order | Military |