Capecitabine and Oxaliplatin or Standard Follow-Up Care in Treating Patients Who Have Undergone Surgery for Locally Advanced Rectal Cancer

DISEASE CHARACTERISTICS: * Histologically confirmed adenocarcinoma of the rectum * Within 15 cm of the anal verge * Locally advanced disease * Underwent complete resection of primary tumor within the past 12 weeks * ypT0-4, N0-2 with definitive histology at surgery * Circumferential resection margin \> 1 mm * No gross evidence of residual disease * Received neoadjuvant fluoropyrimidine-based chemoradiotherapy with ≥ 45 Gy planned total radiation dose, given in 1 of the following fashions: * Prolonged fluorouracil IV during radiotherapy * Low-dose leucovorin calcium and fluorouracil (days 1-5 and 29-33) concurrently with radiotherapy * Oral capecitabine concurrently with radiotherapy * No evidence of metastatic disease PATIENT CHARACTERISTICS: * WHO performance status 0-1 * Absolute neutrophil count ≥ 1,500/mm³ * Platelet count ≥ 100,000/mm³ * Creatinine clearance ≥ 50 mL/min * Bilirubin ≤ 1.25 times upper limit of normal (ULN) * AST and ALT ≤ 1.25 times ULN * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception during and for ≥ 3 months after completion of study treatment * No known dihydropyrimidine dehydrogenase deficiency * No hypersensitivity to platinum compounds * No preexisting peripheral neuropathy ≥ grade 1 * No lack of physical integrity of the upper gastrointestinal tract * No malabsorption syndrome * No other serious uncontrolled medical condition or concurrent medical illness that would compromise life expectancy and/or preclude study compliance, including any of the following: * Serious uncontrolled infections * Significant cardiac disease (e.g., uncontrolled angina, congestive heart failure, cardiomyopathy, or arrhythmias) or myocardial infarction within the past 12 months * Interstitial pneumonia or symptomatic lung fibrosis * No other malignancies except adequately treated in situ carcinoma of the cervix or basal cell or squamous cell carcinoma of the skin, unless disease-free for ≥ 10 years * No history of uncontrolled seizures, CNS disorders, or psychiatric disability that would preclude study compliance PRIOR CONCURRENT THERAPY: * See Disease Characteristics * No prior chemotherapy exceeding 6 weeks in duration * Prior chemotherapy given as part of neoadjuvant treatment (i.e., chemoradiotherapy) may last a maximum of 11-12 weeks * No prior oxaliplatin * Prior mitomycin C, irinotecan hydrochloride, or cetuximab allowed * No concurrent warfarin, antiviral agents, or phenytoin