Although international student assessments and the role of international organisations (IOs) in governing education via an evidence-based educational policy discourse are of growing interest to educational researchers, few have explored the complex ways in which an IO, such as the OECD, gains considerable influence in governing education during the early stages of test production. Drawing on a comparative analysis of the production of two international tests--the Programme for International Student Assessment (PISA) and the Assessment of Higher Education Learning Outcomes (AHELO)--we show how the OECD legitimises its power, and expertise, and defines "what counts" in education. The OECD deploys three mechanisms of educational governance: (1) building on past OECD successes; (2) assembling knowledge capacity; and (3) deploying bureaucratic resources. We argue that the early stages of test production by IOs are significant sites in which the global governance of education is legitimated and enacted.