AO
- AO – (s) Angola (FIPS 10-4 country code; ISO 3166 digram) – (i) Approach Officer (submarine) – Area of Operations
- AOA – (i) Air Officer in charge of Administration – Argonne National Laboratory – (s) Angolan kwanza (ISO 4217 currency code) – (i) Award Of Arms (heraldry)
- AOC – (i) Air Operations Centre – Air Officer Commanding
- AOC in C – Air Officer Commanding in Chief
- AOD – (i) Adrenalin O.D. (punk rock band)
- AOI – (i) Angle Of Incidence (i) Arab Organization for Industrialization
- AOL – (i) America OnLine
- AOR – (i) Area of Responsibility
- AOSTH – (i) Adventures of Sonic the Hedgehog
- AOTS – (i) Attack of the Show!
- AOU – (i) American Ornithologists' Union
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