5 CFR 1820.21: Fees.

(a) Witness fees. OSC may assess fees for attendance by a witness. Such fees will include fees, expenses, and allowances prescribed by the court's rules. If no such fees are prescribed, witness fees will be determined based on 28 U.S.C. 1821, and upon the rule of the federal district closest to the location where the witness will appear. Such fees will include the costs of time spent by the witness to prepare for testimony, in travel, and for attendance in the legal proceeding, plus travel costs.

(b) Payment of fees. A requestor must pay witness fees for current OSC employees and any record certification fees by submitting to the General Counsel a check or money order for the appropriate amount made payable to the United States Department of Treasury.