Open science is becoming a critical topic in the current science policy and research management landscape, in which open access (OA) is seen as the first necessary step forward towards an open work environment in modern day scholarly life. Given its importance, the inclusion of indicators on OA penetration is a natural step in the 2019 edition of the CWTS Leiden Ranking. Until now, university ranking producers have mostly focused their efforts on reporting regular performance indicators (publications, citations, awards, reputation surveys, etc.), leaving out this important new dimension of scholarly communication. In this post, we describe the methodological approach used for assigning the OA labels in the Leiden Ranking 2019. The Leiden Ranking is the first university ranking to include in a comprehensive manner the uptake of OA publishing by universities worldwide.