federal office vs federal department

federal office and federal department both are nouns.

Word	Noun	Adjective	Verb	Adverb
federal office	Yes	No	No	No
federal department	Yes	No	No	No

As nouns, federal office and federal department are synonyms defined as:

federal office and federal department: a department of the federal government of the United States

Other synonyms of federal office include department of the federal government.

federal office (noun)	federal department (noun)
a department of the federal government of the United States	a department of the federal government of the United States