FO vs field-grade officer

FO and field-grade officer both are nouns.

Word	Noun	Adjective	Verb	Adverb
FO	Yes	No	No	No
field-grade officer	Yes	No	No	No

As nouns, FO and field-grade officer are synonyms defined as:

FO and field-grade officer: an officer holding the rank of major or lieutenant colonel or colonel

Other synonyms of FO include field officer.

FO (noun)	field-grade officer (noun)
an officer holding the rank of major or lieutenant colonel or colonel	an officer holding the rank of major or lieutenant colonel or colonel