CO
CO or variants may refer to:

Chemistry
Carbon monoxide (CO), a colorless, odorless, and tasteless gas
Carbonyl group, composed of a carbon atom double-bonded to an oxygen atom: C=O
Cobalt, a chemical element, symbol Co

Computing and telecommunications
.co (second-level domain), the Internet second-level domain meaning "commercial"
.co, the Internet country code top-level domain (ccTLD) for Colombia
Commitment ordering (CO), a concurrency control technique for databases
Telephone exchange, or central office (CO)

Mathematics
Cofunction, or Co, in trigonometry
Cuboctahedron, a uniform polyhedron

People
Nguyễn Hữu Có (1925–2012), Vietnamese general
Conrado Co (born 1940), Filipino badminton player
Alfredo Co (born 1949), Filipino Sinologist
Atoy Co (born 1951), Filipino actor and basketball coach
Leonard Co (1953–2010), Filipino botanist
Nando Có (born 1973), Bissau-Guinean footballer
Kenedy Có (born 1998), Bissau-Guinean footballer
Samuel S. Co, Filipino politician

Places
CO postcode area, or Colchester postcode area, England
Colombia, ISO 3166-1 country code CO
Colorado, United States postal abbreviation CO
Counties of Ireland, abbreviated Co.

Ranks and titles
C.O., post-nominal initials of members of the congregation of the Oratory of Saint Philip Neri
C/O, cadet officer ranks of the Junior Reserve Officers' Training Corps
Commanding officer (CO), the officer in command of a military unit
Contracting Officer (CO or KO), in the U.S. government
Correctional officer (CO), an officer whose duty it is to safeguard and promote the welfare of inmates
Circle officer  (CO), in the Indian government
Conscientious objector

Transportation
Chemins de fer Orientaux (CO), a former Ottoman railway company
Chesapeake and Ohio Railway (C&O or CO), a former American railroad
Cobalt Air (IATA airline designator CO), a Cypriot airline from 2016
Continental Airlines (IATA airline designator CO), an American airline before 2012

Other uses
co-, an English prefix meaning coming together
Cardiac output (CO), the volume of blood being pumped by the heart per time unit
Castres Olympique, a French rugby club
Certificate of occupancy (CO), a legal document
Certificate of origin (often abbreviated to C/O or CoO), in international trade
Company, abbreviated co.
Corsican language, ISO 639-1 language code co
℅, a Unicode character in the Letterlike Symbols block
c/o, former name of Gerhardsen Gerner, an art gallery in Berlin

See also
C0 (disambiguation), the letter–number combination
Care of (disambiguation)
Co-Co locomotives