The Collaborative International Dictionary of English v.0.48:

main office \main office\ n.
   The office that serves as the administrative center of an
   enterprise,

   Syn: headquarters, central office, home office.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

main office
    n 1: (usually plural) the office that serves as the
         administrative center of an enterprise; "many companies
         have their headquarters in New York" [syn: headquarters,
         central office, main office, home office, home
         base]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "main office":
   CP, GHQ, HQ, base, base of operations, booking office, box office,
   branch, branch office, cabinet, center of authority,
   central administration, central office, central station, chambers,
   chancellery, chancery, closet, command post, company headquarters,
   consulate, corporate headquarters, den, embassy, executive office,
   general headquarters, headquarters, home office, legation, office,
   seat, shop, study, ticket office