Inclusion Criteria:

          -  Participant has histologically confirmed diagnosis of high-grade predominantly serous,
             endometrioid, carcinosarcoma, mixed mullerian with high-grade serous component, clear
             cell, or low-grade serous OC, primary peritoneal cancer, or fallopian tube cancer will
             be enrolled in this study (only up to 8 patients with clear cell carcinoma will be
             included and mucinous carcinoma will not be included).

          -  Participant has received 2 previous courses of platinum-containing therapy, and has
             disease that was considered platinum sensitive following the penultimate (next to
             last) platinum course (more than 6 months' period between penultimate platinum regimen
             and progression of disease

          -  Participant has responded to last the platinum regimen (complete or partial response),
             remains in response and is enrolled on study within 8 weeks of completion of the last
             platinum regimen

          -  Participant is able to provide a newly obtained core or excisional biopsy of a tumor
             lesion for prospective testing of BRCA 1/2 and PD-L1 status prior to enrollment

          -  Female participants who are at least 20 years of age and Eastern Cooperative Oncology
             Group (ECOG) performance status of 0 or 1

          -  Participant has adequate organ function as defined in the following:

               1. ANC≥1500/µL

               2. PLT≥100 000/µL

               3. Hemoglobin≥9.0 g/dL or ≥5.6 mmol/L

               4. Creatinine≤1.5 × ULN or calculated creatinine clearance≥30 mL/min for participant
                  with creatinine levels >1.5 × institutional ULN

               5. Total bilirubin≤1.5 ×ULN OR direct bilirubin ≤ULN for participants with total
                  bilirubin levels >1.5 × ULN

               6. AST (SGOT) and ALT (SGPT)≤2.5 × ULN (≤5 × ULN for participants with liver
                  metastases)

               7. International normalized ratio (INR) OR prothrombin time (PT), Activated partial
                  thromboplastin time (aPTT)≤1.5 × ULN unless participant is receiving
                  anticoagulant therapy as long as PT or aPTT is within therapeutic range of
                  intended use of anticoagulants

        Exclusion Criteria:

          -  Participant has mucinous, germ cell, or borderline tumor of the ovary

          -  Participant has a known or suspected deleterious mutation (germline or somatic) in
             either BRCA1 or BRCA2

          -  Participant has a history of non-infectious pneumonitis that required treatment with
             steroids or currently has pneumonitis

          -  Participant either has myelodysplastic syndrome (MDS)/ acute myeloid leukemia (AML) or
             has features suggestive of MDS/AML

          -  Participant has a known additional malignancy that is progressing or has required
             active treatment within the past 3 years

          -  Participant has known active CNS metastases and/or carcinomatous meningitis

          -  Participant has a known history of active TB (Bacillus Tuberculosis)

          -  Participant 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 [qualitative]
             is detected) infection

          -  Participant has uncontrolled hypertension, defined as systolic > 140 mmHg or diastolic
             > 90 mmHg documented by 2 blood pressure readings taken at least 1 hour apart

          -  Participant has a history of hemorrhage, hemoptysis or active gastrointestinal
             bleeding within 6 months prior to randomization