The European Commission has made great strides in pushing for a more open scholarly publishing system in the past 15 years, and Open Access to publications is now mainstreamed into the European Commission research funding framework. The EC has been supporting a consortium of research funders called cOAlition S that have adopted an Open Access mandate for recipients of their funding called Plan S, which required cOAlition S grantees to publish all peer-reviewed articles resulting from their grant funds in open access (OA). In the five years since Plan S was published, cOAlition S has grown from 12 founding members to 28 and the Plan S Principles have been implemented in the participating funders’ policies. We are now in a position to evaluate the first five years of Plan S and cOAlition S, and reflect on lessons learned and the road ahead. The OA-Advance project will contribute to strengthening cOAlition S as an instrument to advance the European Commission’s and participating funders’ shared vision to deliver full and immediate OA. It will work to achieve this by assessing the impact of Plan S through an independent review, and by providing recommendations on how to build on the achievements of Plan S to support a new vision for the future of scholarly publishing.