OBJECTIVES: Oxycodone is a substitute good for morphine, but much costlier. We investigated the difference in the pattern of opioid use among OECD (Organization for Economic Co-operation and Development) countries, using oxycodone-to-morphine ratio (O/M ratio) as an indicator. We also assessed the potential factors which might have influence on that. METHODS: We described the difference in O/M ratio among OECD countries, using the data on morphine and oxycodone use per capita 2005 INCB (International Narcotic Control Board) Report. Pearson correlation coefficient were calculated to investigate its association with other gross healthcare indicators from 2005 OECD Health data.METHODS: METHODS: RESULTS: Highest O/M ratios were observed from Republic of Korea (5.24), Finland(3.85), US(2.17), and European countries generally showed lower O/M ratio. Increased O/M ratio was associated with low public health care expenditure(r=-0.372, p=0.088), number of health care professionals per capita(r=-0.526, p=0.012 for doctors and r=-0.430, p=0.046 for nurses).   CONCLUSIONS:Pattern of opioid use varied much among OECD countries, and was associated with some aspects of health care system, such as health care financing and health care professional density. Pharmacoeconomic evaluation of oxycodone is warranted to provide evidence for proper use of this costlier drug.