This paper identifies and analyses the legitimation strategies used by the OECD as it expanded its role in global educational governance. Whilst the literature recognises the mainly discursive sources of legitimacy which the OECD derives from its testing regime, especially PISA, what remains unexplored is how exactly it has created the legitimacy to monitor SDG 4 -- an arena where it has not been previously involved. Drawing on Suchman's framework for analysing organisational legitimacy, we identify six strategies. We show how these: were used to promote the OECD's pragmatic, moral, and cognitive legitimacy; progressed over time from low-key passive conformity to active manipulation; and, operated on both an episodic and a continual basis.