CO can stand for:

• The molecular formula of carbon monoxide
• Cardiac output, the volume of blood being pumped by the heart per time unit
• Central office, in telecommunications and telephony
• Certificate of Occupancy, a document stating that a building was inspected and is safe to live in
• Chesapeake and Ohio Railway
• Chief Officer
• Chinese Orchestra
• Colombia, in various country codes
• Colorado (United States postal abbreviations)
• Command
• Commanding officer, military
• Commissioned Officer, military
• Communications Officer
• Como, Lombardia, Italy
• Company
• Compliance Officer (FDA)
• Computer organization
• Conjure One, Canadian music artist
• Conquer Online, a MMORPG PC game
• Conscientious objector, military
• Continental Airlines, IATA airline designator
• Contracting Officer
• Corrections Officer
• Custodial Officer

Co can stand for:

• Cobalt, a ferromagnetic metallic chemical element
• Company (law) (Co.), a general business abbreviation. Co is also a common suffix in company names that are portmanteau words, such as Petco (intended to evoke "Pet company")
• Co (ethnicity), an ethnic group in Southeast Asia
• Cowling number, in physics
• Coccygeal nerve of the spinal cord

co can stand for:

• Corsican language, according to ISO 639, alpha-1
• "check out", Revision Control System command

.co can stand for:

• .co, the Internet top-level domain for Columbia, or the second-level domain within country code domains of countries other than US, e.g. .co.uk/

c/o or ℅ can stand for:

• "care of", used to address a letter when the letter must pass through an intermediary
• complaining of, the medical record of the presenting symptom of a patient