EU is saying that when and if you choose to publish papers based on publicly funded research you must ensure those papers will be publicly available. This means you must budget for any cost associated with it but since your budget are public funds it's basically going to be paid for by the EU in most cases.
The "when and if" means that you can choose not to publish or you can choose to publish after you have sought protection (patent). You can also, as a publisher, have a wait period before research papers become "open access" to, I don't know, be an asshole I guess.
Reference (pdf): http://ec.europa.eu/research/participants/data/ref/h2020/gra...
Edit: The little pdf warning.