29 CFR 2700.63: Evidence; presentation of case.

(a) Relevant evidence, including hearsay evidence, that is not unduly repetitious or cumulative is admissible.

(b) The proponent of an order has the burden of proof. A party shall have the right to present a case or defense by oral or documentary evidence, to submit rebuttal evidence, and to conduct such cross-examination as may be required for a full and true disclosure of the facts.