Under the ERC, grants are awarded from the European Union budget, which have various rules on dissemination of project results and open access. The conditions can be checked in the list of grants and their open access rules. In general, open access is mandatory for those grants that have the Article 29 in the contract or tender documents (DISSEMINATION OF RESULTS - OPEN ACCESS - VISIBILITY OF EU FUNDING). It also addresses access to research data. In cases when open access is not mandatory, the ERC strongly recommends it.
General information on ERC grants can be found on the website of the European Centre of Charles University.