Part 2: The second part of this study is a randomized, placebo-controlled, double-blind, parallel study to evaluate the safety, efficacy, and pharmacodynamics of up to 2 dose levels of omaveloxolone (RTA 408) in patients with mitochondrial myopathies. Eligible patients in Part 2 will be randomized 1:1:1 to receive omaveloxolone (RTA 408) (at one of 2 doses chosen from Part 1), or placebo.