• 3 defintions found in Acronymia,
• 5 defintions found in AcroLexic: Acronyms and Abbreviations Dictionary:

OAO

orbiting astronomical observatory

Ontario Association of Orthodontists

orthogonalized atomic orbital

AcroLexic: Dictionary of Acronyms and Abbreviations contains 2 more definitions of “OAO”.

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See also