Pembro With Radiation With or Without Olaparib

Inclusion Criteria: * Male participants with histologically confirmed adenocarcinoma of the prostate * High-risk / very high-risk status per NCCN guidelines * ECOG performance status 0 to 1 * Regional lymph nodes are allowed. * Agree to use contraception during the treatment period and for at least 120 days after the last dose of study intervention and refrain from donating sperm during this period. * Ability to understand and the willingness to sign a written informed consent document. * Adequate organ and marrow function * Prior pharmacologic androgen ablation for prostate cancer is allowed only if the onset of androgen ablation is ≤90 days prior to the date of registration * Known history or current symptoms of cardiac disease, or history of treatment with cardiotoxic agents, should have a clinical risk assessment of cardiac function using the New York Heart Association Functional Classification. Exclusion Criteria: * Prior hormonal therapy with LHRH agonists (e.g., Lupron) and LHRH antagonists (e.g., Degarelix)for prostate cancer continuously for more than 90-days prior to study enrollment. * Prior radiation to the prostate or pelvic nodes radiation. * Previous or major surgery (colorectal anastomosis, total cystectomy, etc.). * History of Ulcerative proctitis. * Concurrent active, additional malignancy in the last 2 years. * Prior systemic anti-cancer therapy including investigational agents within 4 weeks prior to randomization. * Patients with M1 disease