| Acronym | Meaning |
| AO | (USN Rating) Aviation Ordnanceman |
| AO | Abgabenordnung (German tax regulations) |
| AO | Accommodation Officer |
| AO | Accredited Organization |
| AO | ACCS (Army Command and Control System) OSI (Operator System Interface) |
| AO | Acousto Optics |
| AO | Acquisition Objective |
| AO | Action Officer |
| AO | Administration Officer |
| AO | Administrative Office/ Officer |
| AO | Administrative Order |
| AO | Advisory Opinion |
| AO | Aerial Observer |
| AO | Aeronautical Orders (orders authorizing a military person to perform duties in flight) |
| AO | Aerosol Obscurant |
| AO | Afrique Orientale (German East Africa) |
| AO | Agent Orange (herbicidal warfare) |
| AO | Agreement Officer (US AID) |
| AO | Air Observer |
| AO | Air Officer |
| AO | Air Operation |
| AO | Aircraft Operator |
| AO | Airdrome Officer |
| AO | Amphibious Operation |
| AO | And Others |
| AO | Annotated Outline |
| AO | Announcement of Opportunity |
| AO | Appeals Office(r) |
| AO | Arctic Ocean |
| AO | Area of Operation |
| AO | Assessing Officer (tax; India) |
| AO | Assignment Officer |
| AO | Associate Post Office (also called associate office) |
| AO | Asylum Officer (US DHS) |
| AO | Atlantic Ocean |
| AO | Attack Objective |
| AO | Australian Airlines (IATA airline code) |
| AO | Authorizing Official (for IMPAC Purchases) |
| AO | Autonomous Okrug |
| AO | Autres Objets (Canada universal postal union) |
| AO | Auxiliary Oiler (Ship Type) |
| AO | Aviation Officer |
| AO | Aviation Ordnance Person (US DoD) |
| AO | Awards and Obligations |
| AO | Office of the Administrator |
| AO | Operational Availability |
| AO | Order of Australia |
| AO | Other Articles (US postal service) |