• Patients must have dated and signed an approved written informed consent form. This must be obtained before the performance of any protocol-related procedures that are not part of normal patient care.

• Patients must be willing and able to comply with scheduled visits, treatment schedule, laboratory tests and other requirements of the study,

• Target Population

• Inoperable, advanced, or metastatic gastric cancer or gastroesophageal junction or distal esophageal carcinoma and histologically confirmed predominant adenocarcinoma,

• Expression of PD-L1 with a combined positive score (PD-L1 CPS) ≥5, Note: information must be available at the time of inclusion, the examination will be performed locally in the center and secondarily confirmed centrally,

• No prior systemic cancer treatment given as primary therapy for advanced nonresectable or metastatic disease, Note: if patient received neoadjuvant/adjuvant therapy, this therapy should be completed at least 6 months prior to the diagnosis of metastatic or recurrent disease is made. Palliative radiotherapy is allowed and must be completed 2 weeks prior to randomization,

• At least one measurable lesion as assessed by computed tomography (CT)-scan or magnetic resonance imaging (MRI) according to Response Evaluation Criteria in Solid Tumors (RECIST) v 1.1 and feasibility of repeated radiological assessments; radiographic tumor assessment should be performed within 28 days prior to randomization,

• Eastern Cooperative Oncology Group performance status (ECOG PS) of 0 or 1,

• Adequate hematologic and end-organ function, defined by the following laboratory test results, obtained within 14 days prior to randomization of study treatment:

∙ White blood cell ≥ 2000/μL;

‣ Neutrophils ≥ 2000/μL;

‣ Platelets ≥ 100.000/μL;

‣ Hemoglobin ≥ 9.0 g/dL;

‣ Serum albumin ≥ 30 g/L;

‣ Serum creatinine level ≤ 150 μM and calculated creatinine clearance (Cockcroft-Gault) \> 50 mL/minute,

‣ Total bilirubin ≤ 1.5 x upper normal limit (ULN);

‣ Alanine aminotransferase (ALT) ≤ 3.0 x ULN (or ≤ 5.0 x ULN if liver metastases are present);

‣ Aspartame aminotransferase (AST) ≤ 3.0 x ULN (or ≤ 5.0 x ULN if liver metastases are present);

∙ Potassium ≥ 1.0 x lower limit of normal (LLN),

∙ Magnesium ≥ 1.0 x LLN,

∙ Calcium ≥ 1.0 x LLN,

• Baseline-corrected QT interval ≤ 450 msec for males and ≤ 470 msec for females,

⁃ Availability of a representative tumor tissue specimen for exploratory translational research; tumor tissue samples, either formalin- fixed paraffin-embedded (FFPE) tissue block or unstained tumor tissue sections (minimum of 20 positively charged slides) from primary or metastatic site must be submitted to the central laboratory,

⁃ Registration in a national health care system (PUMa-Protection Universelle Maladie included.

⁃ Age and reproductive status

⁃ Age ≥ 18 years,

⁃ Women must not be pregnant, breastfeeding, or expecting to conceive during the study,

⁃ Reproductive status:

• Women of childbearing potential (WOCBP) must have a negative serum pregnancy test within 72 hours prior to the start of study drug,

∙ WOCBP must agree to use an adequate method of contraception or birth control for the duration of study treatment and 5 months (nivolumab), 9 months (oxaliplatin), 6 months (5-FU) or at least 1 month (EXL01) of the patient's last dose of the study drug,

∙ Males who are fertile and sexually active with WOCBP must agree to follow instructions for method(s) of contraception for the duration of study treatment and 6 months (nivolumab, oxaliplatin, or 5-FU) or at least 1 month (EXL01) after the last dose of study treatment. In addition, males must be willing to refrain from sperm donation during this time,