Purpose: The risk of opioid-related aberrant behavior (OAB) in Korean cancer patients has not been previously evaluated. The purpose of this study was to investigate the Opioid Risk Tool (ORT) in Korean cancer patients receiving opioid treatment.Methods: Data were obtained from a multicenter, cross-sectional, nationwide observational study regarding breakthrough cancer pain. The study was conducted in 33 South Korean institutions from March 2016 to December 2017. Patients were eligible if they had cancer-related pain within the past 7 days, which was treated with strong opioids in the previous 7 days.Results: We analyzed ORT results of 947 patients. Only one patient in each sex (0.2%) was classified as high risk for OAB. In patients with an ORT score of 1 or higher, the score primarily represented positive responses for personal history of depression, personal or family history of alcohol abuse, or 16–45 years age range. These patients had more severe average and worst pain scores and used rescue analgesics more frequently than patients with ORT scores of 0. The proportion of moderate- or high-risk patients according to ORT was lower in patients receiving low doses of long-acting opioids than in those receiving high doses (2.0% vs. 6.6%, p = 0.030). Moderate or high risk was more frequent when ORT was completed in an isolated room than in an open, busy place (2.7% vs. 0.6%, p = 0.091).Conclusion: The risk of OAB measured by ORT was very low in cancer patients receiving strong opioids for analgesia.