Pembro With Radiation With or Without Olaparib

This trial is testing whether adding pembrolizumab (an immunotherapy) and/or olaparib (a targeted PARP inhibitor) to standard radiation therapy improves outcomes for men with high-risk or very high-risk prostate cancer that has not yet spread beyond the region. The goal is to improve cure rates while comparing the different drug combinations. **You may be eligible if...** - You are a male with prostate cancer confirmed by biopsy - Your cancer is classified as high-risk or very high-risk by standard guidelines - Your cancer has not spread to distant organs (no M1 disease) - You are in good general health (ECOG 0 or 1) - Prior hormone therapy is allowed only if it started within 90 days before registering - Your organ function meets the required levels **You may NOT be eligible if...** - You have been on hormone-blocking therapy (LHRH agonists or antagonists) continuously for more than 90 days before enrollment - You have had prior radiation to the prostate or pelvis - You have had major prior pelvic surgery - You have ulcerative proctitis (inflammation of the rectum) - You have another active cancer - Your cancer has spread to distant sites (M1 disease) Talk to your doctor to see if this trial is right for you.