The Office of the Secretary of Defense (OSD) is the principal staff element of the Secretary of Defense in the exercise of policy development, planning, resource management, fiscal, and program evaluation responsibilities. OSD includes the immediate offices of the Secretary and Deputy Secretary of Defense, Under Secretaries of Defense, Director of Defense Research and Engineering, Assistant Secretaries of Defense, General Counsel, Director of Operational Test and Evaluation, Assistants to the Secretary of Defense, Director of Administration and Management, and such other staff offices as the Secretary establishes to assist in carrying out assigned responsibilities.

Major elements of OSD (listed alphabetically) and direct links to their home pages (where available):

• Assistant Secretary of Defense for Health Affairs (HA)
• Assistant Secretary of Defense for Homeland Defense (HD)
• Assistant Secretary of Defense for International Security Affairs (ISA)
• Assistant Secretary of Defense for Legislative Affairs (LA)
• Assistant Secretary of Defense for Networks and Information Integration/Chief Information Officer (NII/DoD CIO)
• Assistant Secretary of Defense for Public Affairs (PA)
• Assistant Secretary of Defense for Reserve Affairs (RA)
• Assistant Secretary of Defense for Special Operations and Low Intensity Conflict (SO/LIC)
• Assistant to the Secretary of Defense for Intelligence Oversight (IO)
• Assistant to the Secretary of Defense for Nuclear & Chemical & Biological Defense Programs
• Deputy Secretary of Defense
• Deputy Under Secretary of Defense for Advanced Systems and Concepts
• Deputy Under Secretary of Defense for Civilian Personnel Policy
• Deputy Under Secretary of Defense for Equal Opportunity
• Deputy Under Secretary of Defense for Industrial Policy
• Deputy Under Secretary of Defense for Installations and Environment
• Deputy Under Secretary of Defense for Logistics and Material Readiness
• Deputy Under Secretary of Defense for Military Community and Family Policy
• Deputy Under Secretary of Defense for Military Personnel Policy
• Deputy Under Secretary of Defense for Personnel and Readiness
• Deputy Under Secretary of Defense for Program Integration
• Deputy Under Secretary of Defense for Readiness
• Deputy Under Secretary of Defense for Science and Technology
• Director of Administration and Management (DA&M)
• Director of Defense Research and Engineering (DDRE)
• Director of Force Transformation
• Director of Net Assessment
• Director of Operational Test and Evaluation (DOT&E)
• Director of Program Analysis and Evaluation (PA&E)
• Executive Secretary of the Department of Defense
• General Counsel of the Department of Defense
• Inspector General of the Department of Defense
• Secretary of Defense
• Under Secretary of Defense Comptroller/Chief Financial Officer (USD(C)/CFO)
• Under Secretary of Defense for Acquisition, Technology and Logistics (AT&L)
• Under Secretary of Defense for Intelligence
• Under Secretary of Defense for Personnel and Readiness (P&R)
• Under Secretary of Defense for Policy

Other OSD resources and references:

• Biographies of many OSD leaders, from DefenseLINK's biographies page.
• Functions of OSD, from the DoD Organization and Functions Guidebook.
• Mailing addresses for many OSD leaders, from DefenseLINK's questions section.
• Names, titles and thumbnail portraits of the key OSD leaders, from the Defense Almanac.
• Organizational chart of OSD (.pdf, 10K), also from the DoD Organization and Functions Guidebook.
• Portraits of many OSD leaders, from DefenseLINK's news photo section.