Josh Birchfield Security Contractor Oversight Improvement Act - Directs the Secretary of Defense (DOD) to establish a Quality Assurance Surveillance Plan setting standards to be incorporated in DOD oversight plans governing all security contractors operating in Afghanistan, and other future contingency operations, under a contract or subcontract funded by DOD. Requires: (1) the plan to be implemented within six months after the enactment of this Act, and (2) the Comptroller General to assess DOD compliance with the plan and report assessment results to Congress.