P.O. vs petty officer

P.O. and petty officer both are nouns.

Word	Noun	Adjective	Verb	Adverb
P.O.	Yes	No	No	No
petty officer	Yes	No	No	No

As nouns, P.O. and petty officer are synonyms defined as:

P.O. and petty officer: a noncommissioned officer in the Navy or Coast Guard with a rank comparable to sergeant in the Army

Other synonyms of P.O. include PO.

P.O. (noun)	petty officer (noun)
a noncommissioned officer in the Navy or Coast Guard with a rank comparable to sergeant in the Army	a noncommissioned officer in the Navy or Coast Guard with a rank comparable to sergeant in the Army