N.Y.C. Charter § 1102: Section 1102. Organization of department.

a. Any head of an administration or a department established by this charter, to the extent to which the organization of the administration or department is not prescribed by law, shall by instrument in writing filed in the agency organize the administration or department into such divisions, bureaus or offices and make such assignments of powers and duties among them, and from time to time change such organization or assignments, as the head of the administration or department may consider advisable.

b. Except as provided in section eleven, where divisions, bureaus or offices have been established by law, the mayor may consolidate any two or more divisions, bureaus or offices in any agency under the jurisdiction of the mayor and change the duties of any such division, bureau or office and in like manner reverse or modify any such action.