The Research and Innovation portal of the European Commission has published very useful guidelines about Open Access and how it affects all sorts of H2020 projects. Using simple language, direct messages and revoking misconceptions, the page explains the main ways to Open Access and why the EU advocates for it:

Broader access to scientific publications and data therefore helps to:
build on previous research results (improved quality of results)
encourage collaboration and avoid duplication of effort (greater efficiency)
speed up innovation (faster progress to market means faster growth)
involve citizens and society (improved transparency of the scientific process).
This is why the EU wants to improve access to scientific information and to boost the benefits of public investment in research funded under Horizon 2020.

For a visual representation of the importance of Open Access, check this infographic:

FOSTER (Facilitate Open Science Training for European Research) is an FP7 project that aims at providing a broad training programme to help various stakeholders (such as researchers, postgraduates or librarians) to incorporate Open Access approaches into their practices. The FOSTER project and its portal for Open Access resources and training is included in the HEIRRI Database.