Adjuvant De-Escalated Radiation + Adjuvant Nivolumab for Intermediate-High Risk P16+ Oropharynx Cancer

Inclusion Criteria: * Age \>/= 18 years. * ECOG performance status of 0 or 1. * Patients must have newly diagnosed, histologically or cytologically confirmed squamous cell carcinoma or undifferentiated carcinoma of the oropharynx. Patients must have been determined to have resectable oropharyngeal disease. Patients with primary tumor or nodal metastasis fixed to the carotid artery, skull base or cervical spine are not eligible. * Patients must have intermediate risk factors, as described below as determined by imaging studies (performed \< 45 days prior to registration) and complete neck exam, from the skull base to the clavicles. The following imaging is required: CT scan of neck only with IV contrast or MRI. PET scan of HN and chest with IV contrasted CT correlation is encouraged prior to enrollment. Intermediate risk features: Tobacco \<10 pk-yr: T0-3 plus any one of the following: \>N2b (\> 5 LN's +), N2c/N3, +ENE \>1 mm, or + margin (if approved by surgical chair) OR Tobacco \>10 pk-yr: T0-3 plus any one of the following: any N2, N3, +ENE \>1 mm, or + margin (if approved by surgical chair) * Patients must have no evidence of distant metastases (M0) * Patients must have biopsy-proven p16+ oropharynx cancer; the histologic evidence of invasive squamous cell carcinoma may have been obtained from the primary tumor or metastatic lymph node. It is required that patients have a positive p16 IHC (as surrogate for HPV) status from either the primary tumor or metastatic lymph n…