This study is being done to evaluate if maintenance nivolumab following definitive therapy with radiation and chemotherapy (cisplatin) results in significant improvement in overall survival for patients with intermediate risk HPV positive oropharynx cancer. We are doing this study because we want to find out if this approach is better or worse than the usual approach for HPV positive oropharynx cancer. 
   
The usual approach for patients who are not in a study is treatment with surgery, chemotherapy, and radiation therapy. Usually, combinations of these treatments are used. Cisplatin is the most commonly used chemotherapy drug to use with radiation therapy and is approved by the U.S. Food and Drug Administration (FDA) for this type of cancer. The usual approach for patients who are not in a study includes monitoring the effect of treatment by taking pictures of their tumor(s) with a PET/CT scan at 12 weeks after radiation, and CT scans every 3 months for 3 years after radiation therapy.
   
Participants in this study will get chemotherapy weekly for up to 7 weeks and radiation daily excluding weekends, for up to 7 weeks. Then they will either get nivolumab for up to 12 months and be monitored and observed (group 1), or will be monitored and observed without getting nivolumab (group 2). Nivolumab is given intravenously and is not approved by the FDA for the treatment of this disease.

If the participant's cancer grows during observation, they will then be offered nivolumab for up to 12 months. Participants will be evaluated every 3-6 months for 3 years after treatment, and once a year after that. This means that total study participation may last up to 10 years after treatment.