field officer vs FO

field officer and FO both are nouns.

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

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

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

Other synonyms of field officer include field-grade officer.

field 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