AO (originally American Optical Company) eyeglass lenses, a brand now merged with Carl Zeiss Vision
AO Foundation (originally Arbeitsgemeinschaft für Osteosynthesefragen), a non-profit organisation dedicated to the treatment of trauma and disorders of the musculoskeletal system
AO World, a UK retailer of household appliances trading under the brand name ao.com
Aktsionernoye Obschestvo or Aktsionernoye Obshchestvo (акционерное общество), a type of Russian corporate entity; see open joint-stock company
Athletics Ontario, formally Ontario Track and Field Association (OTAF)