Accordingly to Article 29.2 of your project’s Grant Agreement, each Horizon 2020 beneficiary must ensure open access to peer-reviewed scientific publications relating to results.
The EC just released an useful infograph about the rules and conditions
http://ec.europa.eu/research/participants/data/ref/h2020/other/hi/oa-pilot/h2020-infograph-oa-sci-publ_en.pdf