In this open-label, multicenter, dose-escalation, Phase 1b/2 study, patients who qualify will receive omaveloxolone (RTA 408) at the assigned dose level in combination with ipilimumab or nivolumab. Patients will receive omaveloxolone (RTA 408) orally once daily for 1 week prior to initiation of ipilimumab or nivolumab. For patients treated with ipilimumab , the run-in period will be followed by omaveloxolone (RTA 408) orally once daily in combination with ipilimumab administered at Weeks 1, 4, 7, and 10. After Week 10, patients will receive maintenance treatment with omaveloxolone (RTA 408) alone once daily. For patients treated with nivolumab, the run-in period will be followed by omaveloxolone (RTA 408) orally once daily in combination with nivolumab administered approximately every two weeks as clinically indicated. Each patient will continue at the assigned omaveloxolone (RTA 408) dose level until disease progression occurs, toxicity requiring discontinuation from study drug (i.e., RTA 408) is experienced, the patient has completed approximately 72 weeks of treatment, the patient is discontinued from the study drug for another reason, or the patient withdraws consent. Patients will return 4 weeks after omaveloxolone (RTA 408) treatment completion for a follow-up visit.