Oco or OCO may refer to:
- Oco, a town in Navarre, Spain
- O.co Coliseum in Oakland, California, United States
- Old Cornish language (ISO 639-3 code: oco)
- Online channel optimisation, particularly the use of tools such as A/B testing and marketing automation to improve websites' KPIs.
- Oort cloud object, distant bodies of the solar system
- Orbiting Carbon Observatory-2, successfully launched in 2014, or the original Orbiting Carbon Observatory, lost to a launch vehicle failure in 2009.
- Overseas Contingency Operation, a name for the United States War on Terror
- Calgary Olympic Committee, the local organising committee of the 1988 Winter Olympics
- One cancels other, particularly the use of OCO orders for buying and selling stocks and options on a contingent basis
- Carbon dioxide, with structural formula OCO (simplified to CO)