Inclusion Criteria:  Male/female participants must be at least 18 years of age on the day of signing informed consent and have a histologically confirmed diagnosis of colorectal cancer. Have an unresectable locally-advanced or metastatic colorectal cancer and have progressive disease confirmed by radiologic assessment. Have provided an archival tumor tissue sample or newly obtained core or excisional biopsy of a tumor lesion not previously irradiated. Formalin-fixed, paraffin embedded (FFPE) tissue blocks are preferred to slides. Have DNA HRD defined as either having a BRCA known deleterious mutation (somatic or germinal) and/or RAD 51 score < 10%. Have received at least 2 and no more than 4 prior lines of systemic therapy (including adjuvant treatment). Patients must have received at least: fluoropyrimidines, oxaliplatin and irinotecan, with or without anti-VEGF or anti-EGFR therapy if RAS wild type. Must be oxaliplatin-sensitive defined as having received a minimum of 8 cycles of FOLFOX (fluorouracil, folinic acid and oxaliplatin) or 6 cycles of XELOX (capecitabine and oxaliplatin) as first line therapy, and a progression free survival ≥ 9 months in the first line setting. Patients with both MSS or MSI-H advanced colorectal cancer will be suitable to participate in the trial. The participant (or legally acceptable representative if applicable) provides written informed consent to participate in the trial. Have measurable disease based on RECIST 1.1. Lesions situated in a previously irradiated area are considered measurable if progression has been demonstrated in such lesions after radiotherapy. Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1. Evaluation of ECOG is to be performed within 7 days prior to the first dose of study intervention. Have adequate organ function. Blood samples must be collected within 7 days prior to the start of study intervention. A male participant must agree to use a contraception as detailed in Appendix 3 of this protocol during the treatment period and for at least 28 days prior to any study treatment administration plus an additional 180 days after the last dose of study treatment and refrain from donating sperm during this period.  A female participant is eligible to participate if she is not pregnant (see Appendix 3), not breastfeeding, and at least one of the following conditions applies:  Not a woman of childbearing potential (WOCBP) as defined in Appendix 3 OR A WOCBP who agrees to follow the contraceptive guidance in Appendix 3 during the treatment period and for at least 180 days after the last dose of study treatment.  Exclusion Criteria:  Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent, or with an agent directed to another stimulatory or co-inhibitory T-cell receptor (eg, CTLA-4, OX-40, CD137) or with any PARP inhibitor.  Is currently participating in or has participated in a study of an investigational agent or has used an investigational device within 4 weeks prior to the first dose of study intervention.  Participants who have entered the follow-up phase of an investigational study may participate as long as it has been 4 weeks after the last dose of the previous investigational agent. Participation in an observational (non-interventional) study is allowed.  Has received prior systemic anti-cancer therapy including investigational agents within 4 weeks prior to allocation.  Participants must have recovered from all AEs due to previous therapies to ≤Grade 1 or baseline. Participants with ≤Grade 2 neuropathy may be eligible. Participants with endocrine-related AEs Grade ≤2 requiring treatment or hormone replacement may be eligible If the participant had major surgery, the participant must have recovered adequately from the procedure and/or any complications from the surgery prior to starting study intervention. Has received prior radiotherapy within 2 weeks of start of study intervention. Participants must have recovered from all radiation-related toxicities, not require corticosteroids, and not have had radiation pneumonitis. A 1-week washout is permitted for palliative radiation (≤2 weeks of radiotherapy) to non-CNS disease.  A WOCBP who has a positive urine pregnancy test within 72 hours prior to allocation (see Appendix 3). If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required.  -In the event that 72 hours have elapsed between the screening pregnancy test and the first dose of study treatment, another pregnancy test (urine or serum) must be performed and must be negative in order for subject to start receiving study medication.  Has received a live vaccine or live-attenuated vaccine within 30 days prior to the first dose of study drug. Administration of killed vaccines is allowed. Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy (in dosing exceeding 10 mg daily of prednisone equivalent) or any other form of immunosuppressive therapy within 7 days prior to the first dose of study drug. Has a known additional malignancy that is progressing or has required active treatment within the past 3 years. Participants with basal cell carcinoma of the skin, squamous cell carcinoma of the skin or carcinoma in situ (eg, breast carcinoma, cervical cancer in situ) that have undergone potentially curative therapy are not excluded. Has known CNS metastases and/or carcinomatous meningitis. Has severe hypersensitivity (≥Grade 3) to pembrolizumab or olaparib and/or any of its excipients. Has 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 and is allowed. Has a history of (non-infectious) pneumonitis/interstitial lung disease that required steroids or has current pneumonitis/interstitial lung disease of any etiology. Has an active infection requiring systemic therapy. Has a known history of Human Immunodeficiency Virus (HIV) infection. No HIV testing is required  Has a known history of Hepatitis B (defined as Hepatitis B surface antigen [HBsAg] reactive) or known active Hepatitis C virus (defined as HCV RNA is detected) infection.  No testing for Hepatitis B and Hepatitis C is required unless mandated by local health authority.  Has a history or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the study, interfere with the participant's participation for the full duration of the study, or is not in the best interest of the participant to participate, in the opinion of the treating investigator. Patients unable to swallow orally administered medication and patients with gastrointestinal disorders likely to interfere with absorption of the study medication. Current, clinically relevant bowel obstruction, including sub-occlusive disease, related to underlying disease or any other non-reversible cause. Patient has any known history or current diagnosis of myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML). Has 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 study, starting with the screening visit through 180 days after the last dose of trial treatment. Has had an allogenic tissue/solid organ transplant. Other severe acute or chronic medical condition, or laboratory abnormality that may increase the risk associated with study participation or study drug administration, or may interfere with the interpretation of study results, and in the judgment of the investigator would make the patient inappropriate for study entry.