Study of Pembrolizumab With Maintenance Olaparib or Maintenance Pemetrexed in First-line (1L) Metastatic Nonsquamous Non-Small-Cell Lung Cancer (NSCLC) (MK-7339-006, KEYLYNK-006)

Inclusion Criteria: 1. Have a histologically or cytologically confirmed diagnosis nonsquamous NSCLC. 2. Have stage IV nonsquamous NSCLC. 3. Have confirmation that epidermal growth factor receptor (EGFR), anaplastic lymphoma kinase (ALK), or Proto-oncogene tyrosine-protein kinase (ROS1)-directed therapy is not indicated. 4. Have measurable disease based on RECIST 1.1. 5. Have provided archival tumor tissue sample or newly obtained core or incisional biopsy of a tumor lesion not previously irradiated. Note: Adequacy of biopsy specimen for the above analyses must be confirmed by the central laboratory before the participant can start the induction phase. Submission of another tumor specimen may be required prior to enrolling the participant, if adequate tumor tissue was not provided the first time. 6. Have a life expectancy of at least 3 months. 7. Have a performance status of 0 or 1 on the Eastern Cooperative Oncology Group (ECOG) Performance Status assessed within 7 days prior to the administration of study intervention. 8. Have not received prior systemic treatment for their advanced/metastatic NSCLC. 9. Have adequate organ function. 10. Male and female participants who are not pregnant and of childbearing potential must follow contraceptive guidance during the treatment period and for 180 days afterwards. 11. Male participants must refrain from donating sperm, and female participants must refrain from donating eggs to others or freeze/store for her own use during the treatment period and for 180 days afterwards. Exclusion Criteria: 1. Has predominantly squamous cell histology NSCLC. 2. Has a known additional malignancy that is progressing or has progressed within the past 3 years requiring active treatment. 3. Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis. 4. Has a severe hypersensitivity (≥Grade 3) to pembrolizumab and/or any of its excipients. 5. Has a known hypersensitivity to any components or excipients of cisplatin, carboplatin, pemetrexed, or olaparib. 6. Has an active autoimmune disease that has required systemic treatment in past 2 years. 7. Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy. 8. Has a known history of human immunodeficiency virus (HIV) infection, a known history of hepatitis B infection, or known active hepatitis C virus infection. 9. Has interstitial lung disease, or history of pneumonitis requiring systemic steroids for treatment. 10. Has received prior therapy with olaparib or with any other polyadenosine 5' diphosphoribose (polyADP ribose) polymerization (PARP) inhibitor. 11. Has received prior therapy with an agent directed to programmed cell death ligand 1 (PD-L1), anti PD-L2, or directed to a stimulatory or co-inhibitory T-cell receptor (e.g., cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), OX-40, CD137). 12. Has myelodysplastic syndrome (MDS)/acute myeloid leukemia (AML) or with features suggestive of MDS/AML. 13. Has history of a second malignancy, unless potentially curative treatment has been completed with no evidence of malignancy for 2 years. 14. Has completed palliative radiotherapy within 7 days of the first dose. Participants must have recovered from all radiation-related toxicities and not require corticosteroids.