Additional Study Information: The purpose of this study is to see if ONC201 is effective in treating participants with glioma when given following radiation therapy, how safe and tolerable ONC201 as compared to using placebo (a capsule that looks like ONC201 but does not contain any active drug), and to see how well ONC201 can improve quality of life and brain functioning in patients with glioma. ONC201 is a new drug that is being developed for treating people with glioma. ONC201 may help to remove tumor cells without affecting normal cells of the body. ONC201 is a first-in-class drug, which means it is a new drug having unique action, and it is not approved by the U.S. Food and Drug Administration (FDA) for treating glioma.