Oxaliplatin and S-1 or Oxaliplatin and Capecitabine in Treating Patients With Recurrent, Metastatic, or Unresectable Gastric Cancer

DISEASE CHARACTERISTICS: * Histologically confirmed adenocarcinoma of the stomach * Unresectable advanced disease or recurrent disease after resection * At least one radiographically documented (CT scan or MRI) measurable or evaluable lesion in a previously non-irradiated area according to RECIST * No clinical evidence of brain metastases or history of other CNS disease unless adequately treated PATIENT CHARACTERISTICS: * ECOG performance status 0-2 * Estimated life expectancy \> 3 months * Hemoglobin ≥ 9 g/dL * White blood cell ≥ 4,000/µL * ANC ≥ 2,000/µL * Platelets ≥ 100,000/µL * Bilirubin ≤ 1.25 times upper limit of normal (ULN) (≤ 2.0 times ULN if hepatic metastasis present) * Serum creatinine ≤ 1.5 times ULN * Creatinine clearance ≥ 60 mL/min * AST/ALT ≤ 3.0 times ULN (≤ 5.0 times ULN if hepatic metastasis present) * Alkaline phosphatase ≤ 3.0 times ULN (≤ 5.0 times ULN if bone metastasis present) * Must have an intact gastrointestinal tract * Able to take oral medications * No medically uncontrolled severe infections or complications * No prior malignancy other than gastric cancer in the last 5 years except for basal cell cancer of the skin or preinvasive cancer of the cervix * Not pregnant or nursing * No neuropathy ≥ grade 2 * No clinically relevant heart disease * No evidence of past medical history or psychosocial dysfunction that contraindicates the use of an investigational drug or puts the patient at risk * No dementia, altered mental status, or any psychiatric condition that would prohibit the understanding or rendering of informed consent * No uncontrolled hepatitis B or C, chronic liver disease, or diabetes mellitus * No other evidence of inappropriate suspicious condition PRIOR CONCURRENT THERAPY: * No prior chemotherapy for advanced or recurrent disease * Prior adjuvant chemotherapy allowed if finished \> 6 months before start of study treatment * No prior therapeutic radiotherapy * Prior palliative radiotherapy allowed if it was not done for primary, evaluable, or intraabdominal lesions * No prior capecitabine or oxaliplatin * No other concurrent chemotherapy or radiotherapy (except localized radiotherapy for pain relief) * No concurrent chemically related analogues, such as warfarin, phenytoin, or allopurinol * No concurrent steroid therapy except as follows: * Prophylactic use for hypersensitivity control or antiemetic purpose allowed * Chronic low dose of steroid (less than methylprednisolone 20 mg or equivalent dose) allowed