ARM II: Within 5 weeks after completion of chemoradiation, patients receive temozolomide PO on days 1-5. Treatment repeats every 28 days for up to 6 courses in the absence of disease progression or unacceptable toxicity. Patients also receive nivolumab IV over 60 minutes once every 2 weeks for 16 weeks and then once every 2 weeks for 48 weeks in the absence unacceptable toxicity.