A Study to Investigate the Efficacy and Safety of ONO-4578 in Combination With Nivolumab and Chemotherapy in Chemotherapy-naïve Participants With HER2-negative Unresectable Advanced or Recurrent Gastric Cancer (Including Esophagogastric Junction Cancer)

Inclusion Criteria: * Patients with gastric or esophagogastric junction cancer histologically diagnosed as adenocarcinoma * Patients have not been treated with systemic chemotherapy as first-line therapy * Patients who can provide tumor tissue samples Exclusion Criteria: * Patients unable to take oral medicines * Patients with HER2-positive * Patients with contraindications to nivolumab, oxaliplatin, S-1, or capecitabine * Patients who have a history of severe drug-related adverse reactions caused by non- steroidal anti-inflammatory drugs (NSAIDs) or who have complications caused by NSAIDs requiring treatment * Patients with concurrent autoimmune disease or a history of chronic or recurrent autoimmune disease, such as rheumatoid arthritis * Patients with headache and/or nausea associated with brain metastasis