MO i believe is the abbreviation that you are looking for.
A period .
PO
OK is the abbreviation used.
Post Office!!
GPO
Are you referring to the abbreviation for the state "Maryland"?If so, then it's MD. That IS the "full form".
Post Office!!
It is represented by the letters: FL
Fleet Post Office
P.O. Box 785
P.O. or just PO
You didn't say for what US state- most have 2 letter abbreviations commonly used by post office. On their website, there is a complete list of all US states