Post-Office box vs PO Box

Post-Office box and PO Box both are nouns.

Word	Noun	Adjective	Verb	Adverb
Post-Office box	Yes	No	No	No
PO Box	Yes	No	No	No

As nouns, Post-Office box and PO Box are synonyms defined as:

Post-Office box and PO Box: a numbered compartment in a post office where mail is put to be called for

Other synonyms of Post-Office box include call box, letter box, POB.

Post-Office box (noun)	PO Box (noun)
a numbered compartment in a post office where mail is put to be called for	a numbered compartment in a post office where mail is put to be called for