29 CFR §18.15

1. (a)Substitution during hearing. If the judge is unable to complete a hearing, a successor judge designated pursuant to § 18.12 may proceed upon certifying familiarity with the record and determining that the case may be completed without prejudice to the parties. The successor judge must, at a party's request, recall any witness whose testimony is material and disputed and who is available to testify again without undue burden. The successor judge may also recall any other witness.
2. (b)Substitution following hearing. If the judge is unable to proceed after the hearing is concluded, the successor judge appointed pursuant to § 18.12 may issue a decision and order based upon the existing record after notifying the parties and giving them an opportunity to respond. Within 14 days of receipt of the judge's notice, a party may file an objection to the judge issuing a decision based on the existing record. If no objection is filed, the objection is considered waived. Upon good cause shown, the judge may order supplemental proceedings.