main office vs headquarters

main office and headquarters both are nouns.

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

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

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

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

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