Radiation Therapy and Capecitabine With or Without Oxaliplatin in Treating Patients Who Are Undergoing Surgery for Stage II or Stage III Rectal Cancer

RATIONALE: Radiation therapy uses high-energy x-rays to kill tumor cells. Drugs used in chemotherapy, such as capecitabine and oxaliplatin, 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. Giving radiation therapy together with combination chemotherapy before surgery may shrink the tumor so it can be removed. It is not yet known whether giving radiation therapy together with capecitabine is more effective with or without oxaliplatin before surgery in treating rectal cancer. PURPOSE: This randomized phase III trial is studying radiation therapy, capecitabine, and oxaliplatin to see how well they work compared to radiation therapy and capecitabine in treating patients who are undergoing surgery for stage II or stage III rectal cancer.