5 C.F.R. § 837.103 – Notice.

(a) To OPM. On or before the date a reemployed annuitant is appointed, the appointing agency must notify OPM in writing of the appointment, and provide OPM with the following information—
1. (1) The annuitant's name, date of birth, social security number (if applicable), and retirement claim number;
2. (2) A description of the kind of appointment;
3. (3) Whether the amount of annuity allocable to the period of reemployment is, or will be, withheld from the reemployed annuitant's pay, in accordance with § 837.303 of this part; and
4. (4) When the appointment is an interim appointment under § 772.102 of this chapter, an explicit statement that the appointment is required by the Whistleblower Protection Act of 1989.
(b) To annuitant. The agency should advise the annuitant in writing, generally, of the effect reemployment has on annuitant status and/or the continued receipt of annuity, the possible, future retirement benefits that may be payable to an annuitant on the basis of reemployment, and, for CSRS annuitants, whether the annuitant may elect to have retirement deductions withheld from his or her basic pay.
(c) Obligation of annuitant to provide information. Before appointment, and as a condition of reemployment, the annuitant must provide the employing agency with the following information—
1. (1) Whether the annuitant is then in receipt of annuity;
2. (2) The gross monthly amount of annuity the annuitant is then receiving;
3. (3) Whether the annuitant is a disability annuitant, and if so, whether OPM has found the annuitant recovered from his or her disability, or restored to earning capacity; and.
4. (4) If the annuitant is a CSRS annuitant, whether the annuitant's retirement was based on an involuntary separation, not for charges of misconduct or delinquency.