Optimal Duration of Oxaliplatin in Adjuvant XELOX for Gastric Cancer Patients (EXODOX)

Inclusion Criteria: * Histologically or cytologically confirmed gastric or gastroesophageal junction adenocarcinoma patients who underwent curative surgery (D1 beta or D2 resection) * Pathologically confirmed stage II, III patients (AJCC 8th edition) * Age 19 years and older * Eastern Cooperative Oncology Group (ECOG) performance status 0 or 2 * Adequate marrow function (ANC \> 1,500/uL, Platelet \>100,000/uL, Hb \> 8.0 g/dL, patients with chronic anemia who require intermittent blood transfusions can also participate in the study) * Adequate renal function, with serum creatinine \< 1.5 x upper limit of normal (ULN). * Adequate hepatic function with serum total bilirubin ≤ 1.5 x ULN, alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 3 x ULN * Written, informed consent to the study Exclusion Criteria: * Female patients who are pregnant or breast-feeding * Positive pregnancy test at baseline (postmenopausal women should be amenorrhea for at least 12 months to be considered non-fertile) * Sexually active men and women who are not willing to implement contraception during study and until 3 months after discontinuation of study drug * Evidence of metastasis (including cytologically confirmed malignant ascites) * Prior systemic chemotherapy or radiation therapy for stomach cancer * Patients who have not recovered from serious complications of gastrectomy * History of other malignancies within the last 3 years (excluding adequately treated basal cell carcinoma of the skin, in situ cancer of the cervix, non-metastatic thyroid cancer) * A history of clinically significant uncontrolled seizures, central nervous system disorders, or mental disorders, which make it impossible to understand the informed consent or interfere with compliance with oral drug intake * Clinically significant (i.e., active) heart disease: e.g. unstable angina requiring medication, symptomatic coronary artery disease, congestive heart failure with NYHA grade II or higher, severe cardiac arrhythmias or acute coronary syndrome in the past 6 months (including myocardial infarction) * Lack of integrity or malabsorption syndrome in the upper gastrointestinal tract, which is likely to affect the absorption of study drug * Serious uncontrolled infection or other serious uncontrolled disease * History of allograft requiring immunosuppression therapy * Received any investigational drug or procedure within 4 weeks prior to randomization * Active viral infection (for hepatitis B carrier, patients can be registered if HBV-DNA titer is less than 20,000 IU/mL, and are allowed to use prophylactic antiviral agents by investigator's choice) * Active HIV infection * Patients with peripheral sensory neuropathy with functional impairment