Drugs used in chemotherapy use different ways to stop tumor cells from dividing so they stop growing or die. Radiation therapy uses high-energy x-rays to damage tumor cells. Combining chemotherapy, radiation therapy, and surgery may be a more effective treatment for cancer of the rectum. Phase II trial to study the effectiveness of combining oxaliplatin, fluorouracil, and external-beam radiation therapy followed by surgery in treating patients who have locally advanced cancer of the rectum