5 C.F.R. § 352.205
An appointing officer who intends to employ with reemployment rights an employee of another executive agency shall give the losing agency written notice at least 15 calendar days before the effective date of the proposed action. If the losing agency believes the grant of reemployment rights would be detrimental to the public interest, it may appeal the proposed grant to OPM within 15 calendar days after receipt of the notice. The losing agency, at the same time, shall furnish a copy of the appeal to the prospective appointing officer, who shall withhold the proposed grant pending decision on the appeal. OPM shall determine whether the employee will be given reemployment rights and notify both agencies accordingly. If the losing agency does not appeal within 15 calendar days, the employee shall be granted reemployment rights.
5 C.F.R. § 352.205