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]