MO i believe is the abbreviation that you are looking for.
A period .
PO
OK is the abbreviation used.
GPO
Post Office!!
Post Office!!
Are you referring to the abbreviation for the state "Maryland"?If so, then it's MD. That IS the "full form".
It is represented by the letters: FL
P.O. or just PO
Fleet Post Office
P.O. Box 785
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