ACO, AcO, or Aco may refer to:

• Aco Records
• ACO, C.A., a Venezuelan holding company
• Air Cadet Organisation, collective name for the UK cadet forces sponsored by the Royal Air Force
• Allied Command Operations, the NATO strategic command
• Australian Chamber Orchestra
• Alvarez Chamber Orchestra
• American Composers Orchestra
• American Cornhole Organization
• Association of Cricket Officials
• Automobile Club de l'Ouest, organizers of the 24 Hours of Le Mans auto race
• Canadian Association of Orthodontists (Association canadienne des orthodontistes, ACO)

Acetoxy group, abbreviated AcO or OAc, is a chemical functional group of the structure CH₃-C(=O)-O-. It differs from the acetyl group CH₃-C(=O)- by the presence of one additional oxygen atom. The name acetoxy is the short form of acetyl-oxy.