P.O. vs master-at-arms

P.O. and master-at-arms both are nouns.

Word	Noun	Adjective	Verb	Adverb
P.O.	Yes	No	No	No
master-at-arms	Yes	No	No	No

As nouns, master-at-arms is a hyponym of P.O.; that is, master-at-arms is a word with a more specific, narrower meaning than P.O.:

P.O.: a noncommissioned officer in the Navy or Coast Guard with a rank comparable to sergeant in the Army
master-at-arms: the senior petty officer; responsible for discipline aboard ship

Other hyponyms of P.O. include SCPO, senior chief petty officer.

P.O. (noun)	master-at-arms (noun)
a noncommissioned officer in the Navy or Coast Guard with a rank comparable to sergeant in the Army	the senior petty officer; responsible for discipline aboard ship