headquarters vs main office

headquarters and main office both are nouns.

Word	Noun	Adjective	Verb	Adverb
headquarters	Yes	No	No	No
main office	Yes	No	No	No

As nouns, headquarters and main office are synonyms defined as:

headquarters and main office: (usually plural) the office that serves as the administrative center of an enterprise

Other synonyms of headquarters include central office, home base, home office.

headquarters (noun)	main office (noun)
the military installation from which a commander performs the functions of command	(usually plural) the office that serves as the administrative center of an enterprise
(usually plural) the office that serves as the administrative center of an enterprise
(plural) a military unit consisting of a commander and the headquarters staff