The fight against tax avoidance and tax evasion has become a policy priority of many states. Unilateral action is, however, not an appropriate solution. Global problems of tax avoidance and evasion can only be successfully addressed through international cooperation. In this context, the efforts of the OECD for enhancing cooperation in international taxation have been impressive over the last decade. Reference can, for example, be made to the various initiatives relating to information sharing and the Base Erosion and Profit Shifting (BEPS) Project. Although these initiatives seem legitimate, the role of the OECD in international tax law-making has been criticized. The more influential the OECD’s initiatives are and the more the decision-making authority in relation to taxation shifts from the national to the international level, the more its legitimacy is questioned. The purpose of this article is to examine whether the OECD indeed lacks legitimacy and, if so, what could be done to overcome such a legitimacy deficit. The article starts by analysing in more detail the role that the OECD has played (and is still playing) in international tax law-making and what legal value should be attached to the instruments that have been developed by the OECD. The authors then proceed to assess the legitimacy of the OECD during the BEPS Project. A conceptual framework is needed to assess an international institution’s legitimacy. In this respect, the political science-oriented angle of input, output and throughput legitimacy was taken as a basis. The research shows that the OECD lacked input and output during the BEPS Project, but the authors also identified many flaws in the decision-making process itself. The authors argue that whatever international (rule-making) body is developed to take on a role in international tax law-making, legitimacy issues will remain in the absence of appropriate decision-making procedures. Whilst turning the OECD into a sounding board under the auspices of another organization may therefore not be very helpful, the organization should acknowledge that it needs to monitor its decision-making procedures if it wants to remain the guardian of open tax norms in international taxation. More transparent and equal participation by states, other stakeholders and the public at large, as well as more robust accountability mechanisms, may restore the OECD’s legitimacy.