When managing a European project (and indeed other kind of funding), the publication of your papers in open access format is getting 'mandatory'. In the case of a Marie Curie project, one have different options for publication:
1) 'Gold Open Access'. This is like the 'king' of the Open Access providing the larger freedom to your paper from the moment in which the authors pay for this status. Every editorial has a different process to give 'gold open access' to your paper and every journal has a different fee. In an ideal case, this would be the best option for all the published papers in order to provide fully 'Open Science'. I would suggest to always try this first option for your paper. Check for discounts and help to afford it (from your institution), and it is one of the main publications of your project you may include the costs in the budget. For example, in the case of the Mari Curie, it is possible to ask for 'gold open access' from the management and indirect costs. Otherwise, when writing the project proposal, we have to plan in advance some budget to cover the publication costs of your out coming papers.
2) 'Green Open Access'. This option varies according to the journal. Normally, your paper is open access after a certain embargo period and the journal also provides a number of free accessed pdf files to be shared. This option is generally free of charge.
3) In the case of the Marie Curie project, you have two additional options: OpenAire and Zenodo. Regarding OpenAire, this is an excellent option when the 'green option' does not allow the authors either the self-archiving of the paper after the embargo period or does not provide a number of links to the 'free paper' until the embargo period. When one beneficiary is listing the papers from the project in the 'Participant Portal' it is possible to directly apply to include your paper in OpenAire. In the case of Zenodo, this is a repository specifically targeted to data and publications from EU projects. From my point of view, if OpenAire does not work for your paper this is the best option. You can sing up directly with your ORCID profile.
Anyway, every time that one is confusing about the possibilities to be provided to your paper/research, there is plenty of guidances, information and possibility of help in every editorial, journal and the organisation that gave the funding.
Always helpful information is in the air.
'Open Science is the practice of science in such a way that others can collaborate and contribute, where research data, lab notes and other research processes are freely available, under terms that enable reuse, redistribution and reproduction of the research and its underlying data and methods.'