This is a trial studying patients with human papilloma virus (HPV) positive oropharyngeal cancer with tumors that can be removed via transoral surgery. Following surgery, patients will be classified as either low, intermediate, or high risk based on the characteristics of the tumors. Low risk patients (Arm S) will receive no further treatment after surgery. Intermediate risk patients (Arm RT) will be treated with Intensity Modulated Radiotherapy (IMRT) after surgery. High risk patients (Arm CRT) will receive a combination of IMRT and chemotherapy after surgery. Patients will be followed for up to five years after the completion of treatment.

This phase II trial is designed to rationally de-escalate adjuvant (Intensity Modulated Radiotherapy (IMRT) in the post-transoral surgery (TOS) setting in a study population consisting of patients with resectable oropharynx carcinoma, p16+ as confirmed by immunohistochemistry IHC, with a performance status (PS) of 0-1. Patients will be classified into one of three category/treatment groups (low-, intermediate-, and high-risk) according to their highest pathologically risk feature. Radiation will be given via an IMRT technique. For the high-risk patient group, a reduced, but slightly accelerated radiotherapy (RT) fractionation regimen of 50 Gy (HCC 18-034) in conjunction with cisplatin will be used compared to the standard 66 Gy and cisplatin. Low risk patients will transition to observation, intermediate risk patients will receive 30 Gy in 15 fractions of IMRT, and high risk patients will receive 50 Gy in 25 fractions (one day a week will include two treatments) plus 40 mg/m2 Cisplatin for 5 weeks. Patients who are not able to tolerate cisplatin will receive carboplatin instead.