Phase II Study of Short Course FOLFOX Chemotherapy With Either Nivolumab or Nivolumab + Radiation in the First Line Treatment of Metastatic or Unresectable Gastroesophageal Cancers (BMS Protocol CA209-76L)

Inclusion Criteria * Subjects with a diagnosis of advanced unresectable or metastatic gastroesophageal adenocarcinoma (eg. gastric, gastroesophageal junction, and esophageal adenocarcinoma) * Be willing and able to provide written informed consent/assent for the trial * Age \> 18 years * ECOG performance status ≤ 1 * Absolute neutrophil count ≥ 1,500/mL * Platelets ≥ 100,000/mL * Total bilirubin ≤ 1.5x upper limits of normal, unless the patient has known Gilbert's disease. * AST/ALT ≤ 2.5 upper limits of normal, or \< 5x ULN for subjects with liver metastases * Creatinine ≤ 1.5 mg/dl. If Creatinine \> 1.5 mg/dl, creatinine clearance \> 60 ml/min * Be willing to provide tissue from a newly obtained core or excisional biopsy of a tumor lesion. Newly-obtained is defined as a specimen obtained up to 28 days prior to initiation of treatment on Day 1. * For all males and females of childbearing potential, they should be agreeable to use an adequate method of contraception or birth control. For females of child bearing potential, a negative pregnancy test within 7 days of start of study drug is required Exclusion Criteria * Prior cytotoxic therapy for metastatic or incurable disease. * Patients may have had prior therapy with curative intent for localized disease, if their recurrence or disease progression was more than six months from completing prior therapy. * HER2 positive adenocarcinoma * Diagnosis of immunodeficiency or is receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of trial treatment. * Known history of active TB (Bacillus Tuberculosis) * Known additional malignancy that is active. Exceptions include basal cell carcinoma of the skin or squamous cell carcinoma of the skin that has undergone potentially curative therapy or in situ cervical cancer. * Previous invasive malignancy treated with curative intent less than 3 years from time of registration. Exceptions include prostate cancer or basal cell skin cancer. * Active autoimmune disease that has required systemic treatment in the past 2 years (i.e. with use of disease modifying agents, corticosteroids or immunosuppressive drugs). Replacement therapy (eg., thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency, etc.) is not considered a form of systemic treatment. * Known history of, or any evidence of active, non-infectious pneumonitis. * Active infection requiring systemic therapy. * History or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the trial, interfere with the subject's participation for the full duration of the trial, or is not in the best interest of the subject to participate, in the opinion of the treating investigator. * Known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the trial. * Is pregnant or breastfeeding, or expecting to conceive or father children within the projected duration of the trial, starting with the pre-screening or screening visit through 120 days after the last dose of trial treatment. * Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent. * Known history of Human Immunodeficiency Virus (HIV) (HIV 1/2 antibodies). * Known active Hepatitis B (e.g., HBsAg reactive) or Hepatitis C (e.g., HCV RNA \[qualitative\] is detected). * Has received a live vaccine within 30 days of planned start of study therapy.