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