This clinical trial uses nivolumab and radiotherapy prior to definitive surgical resection of tumors in patients with Head and Neck Squamous Cell Carcinoma (OHNSCC) with the primary objective of determining the safety and feasibility of preoperative immunoradiotherapy. In addition, tumor tissue, microbiome samples, and peripheral blood will be obtained for exploratory immunologic end points including measurements of tumor infiltrating immune cell populations based on flow cytometry and immunohistochemistry as well as circulating immunological parameters. This is the first study to evaluate the safety and efficacy of neoadjuvant radiation + PD-1 blockade in patients with HNSCC.