Here's the sequence put in place by BO that was revoked:
(1) Deputy Secretary of Defense;
(2) Secretary of the Army;
(3) Secretary of the Navy;
(4) Secretary of the Air Force;
(5) Under Secretary of Defense for Acquisition, Technology, and Logistics;
(6) Under Secretary of Defense for Policy;
(7) Under Secretary of Defense (Comptroller);
(8) Under Secretary of Defense for Personnel and Readiness;
(9) Under Secretary of Defense for Intelligence;
(10) Deputy Chief Management Officer, Department of Defense;
(11) Principal Deputy Under Secretary of Defense for Acquisition, Technology, and Logistics;
(12) Principal Deputy Under Secretary of Defense for Policy;
(13) Principal Deputy Under Secretary of Defense (Comptroller);
(14) Principal Deputy Under Secretary of Defense for Personnel and Readiness;
(15) Principal Deputy Under Secretary of Defense for Intelligence;
(16) Director of Defense Research and Engineering;
(17) General Counsel of the Department of Defense, the Assistant Secretaries of Defense, the Assistant to the Secretary of Defense for Nuclear and Chemical and Biological Defense Programs, the Director of Operational Test and Evaluation, the Director of Operational Energy Plans and Programs, and the Director of Cost Assessment and Program Evaluation;
(18) Under Secretaries of the Army, the Navy, and the Air Force; and
(19) Assistant Secretaries of the Army, the Navy, and the Air Force, and General Counsels of the Army, the Navy, and the Air Force.
https://obamawhitehouse.archives.gov/realitycheck/the-press-office/executive-order-providing-order-succession-within-department-defense