There is no definitive, single abbreviation for office. See some following examples:
O - Governmental » FBI Files
OFF - Office Governmental » Military
OFC - Office Governmental » United Nations
OFC - Governmental » NASA
OFC - Miscellaneous » USPS
OFC - Business » General
Source: Abbreviations.com
OK is the abbreviation used.
PO
No known abbreviation.
MO i believe is the abbreviation that you are looking for.
GPO
Post Office!!
The common abbreviation for "office" is "OFC." Another frequently used abbreviation is "Off." These abbreviations are often used in business contexts or informal communication.
The abbreviation 'OED' can stand for many different things including the Office of Economic Development, The Office of Enrollment and Discipline, and Office of the Executive Director.
TSO (The Stationery Office)
Post Office!!
Home Box Office
Office of Management and Budget