FO vs field officer

FO and field officer both are nouns.

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

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

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

Other synonyms of FO include field-grade officer.

FO (noun)	field 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