Office of the Secretary of Defense

The Office of the Secretary of Defense (OSD) is a headquarters-level staff of the United States Department of Defense. It is the principal civilian staff element of the U.S. secretary of defense, and it assists the secretary in carrying out authority, direction, and control of the Department of Defense in the exercise of policy development, planning, resource management, fiscal, and program evaluation responsibilities.

Source: Wikipedia — Office of the Secretary of Defense (CC BY-SA 4.0)