field-grade officer vs FO

field-grade officer and FO both are nouns.

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

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

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

Other synonyms of field-grade officer include field officer.

field-grade officer (noun)	FO (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