petty officer vs P.O.

petty officer and P.O. both are nouns.

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

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

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

Other synonyms of petty officer include PO.

petty officer (noun)	P.O. (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