PISA for Development (PISA-D) was a pioneering pilot project designed to make PISA, which compares the performance of 15-year-olds in school, more suitable for low- and middle-income countries. This would allow the OECD to move beyond its traditional focus on more affluent nations and to play a central role in monitoring the Sustainable Development Goals. PISA-D was declared a success by the OECD, and its most innovative feature was that, unlike PISA, its assessment included out-of-school youth (OOSY). We analyse that strand of the assessment focussing on who was assessed. We argue that the inclusion of OOSY has been misrepresented by the OECD. Building on the literature on the OECD's sources of legitimacy and applying Suchman's framework for analysing organisational legitimacy, we portray its shifting definitions of OOSY as a tactical move that allowed it to ensure the project's success and resolve problems that challenged its sources of legitimacy.