OA

Short definition from Wiktionary

Preposition

1. 1
   Initialism of on or about.

Noun

1. 1
   Initialism of open access.
2. 2
   Initialism of organizing authority.
3. 3
   Initialism of office assistant.
4. 4
   Initialism of output area.
5. 5
   Initialism of online assessment.

Adjective

1. 1
   Initialism of overacting.

Proper noun

1. 1
   Order of the Arrow