RATIONALE: Drugs used in chemotherapy, such as oxaliplatin, S-1, and capecitabine, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Giving more than one drug (combination chemotherapy) may kill more tumor cells. It is not yet known whether giving oxaliplatin together with S-1 is more effective than giving oxaliplatin together with capecitabine.