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 Metal carbonyl, carbon monoxide as a ligand to transition metals 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 Marivic Co-Pilar (born 1971), Filipino politician 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, US 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, 2016-2018), a Cypriot airline Continental Airlines (IATA airline designator CO, 1937-2012), an American airline
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 Co (Armenian letter) Company, abbreviated co. Corsican language (ISO 639-1 language code "co") (), a character in the Unicode Letterlike Symbols block, meaning 'care of' c/o, former name of Gerhardsen Gerner, an art gallery in Berlin
See also C0 (disambiguation), the letternumber combination Care of (disambiguation) Cobalt (disambiguation) Co-Co locomotives