A Clockwork Orange, novel and film

Allied Command Operations, the NATO strategic command

Ant colony optimization algorithm

Animal control officer

An abbreviation for Australian Chamber Orchestra

ACO (musician), J-Pop singer

Aco Records, a record label

Automobile Club de l'Ouest, organizers of the famous 24 Hours of Le Mans auto race

Acetoxy group (AcO), a chemical functional group

Air Cadet Organisation, collective name for the UK cadet forces sponsored by the Royal Air Force

ACO Hardware, a Michigan hardware store