The recent "Open access to scientific peer reviewed publications in Horizon 2020: first data" report by the European Commission on Horizon 2020 reveals an important success rate on Open Access to scientific peer-reviewed publications.
While Horizon2020 is still in its early years, and many projects have not lead yet to publications, data available from two different datasets (CORDA and OpenAIRE) show an Open Access rate between 60 and 70% out of the total articles published.

Within the Horizon 2020 mandate, researchers can choose whether to publish in Open Access journals or to deposit the final copy of their articles in institutional repositories. When publishing in Open Access journals with Article Processing Charges, those costs are eligible for reimbursement.

The deposition of peer-review articles in institutional repositories, on the other side, may imply a period of access embargo to them, depending on the journal policies.
Data from CORDA show the overall costs for APCs and the number of articles published per peer-reviewed journal, with the total expenditure for APC and the average APC per article. 406k € have been spent to cover APCs during the duration of Horizon 2020 so far. The average APC in Horizon 2020 is 1006€ per publication, and there is a small number of high APCs (+4000€).
The same dataset provides data about the embargo period calculated on the available publications from H2020 projects. Most articles are free to read, while 34% of the others have 12 months of embargo - the average embargo period is 11,21 months.

Data provided by OpenAIRE come from a variety of sources that contribute to the numbers of the OpenAIRE portal after deduplication, cleaning, and text-mining processes on the metadata, as well as on the actual content. These data are based on those retrieved from compliant repositories and journals (literature and data), CRIS systems, library databases and end-user feedbacks. To double-check data for publications with DOI (and links to Scimago), OpenAIRE refines them via queries to CrossRef. To date, more than six thousand H2020 publications are available in the OpenAIRE portal, with an average of 23 publications per project. 50% of the H2020 publications available on OpenAIRE are Open Access; considered that 7 are restricted to specific groups and 38 are still in embargo, the OA success rate is of 60.8%.
Per the data provided by the two datasets and shown in the table below, green open access seems to be a more popular choice among Horizon 2020 grantees than gold OA. The average embargo period that may apply to publications deposited in repositories is within the limit of 12 months of the EC mandate (although this period is refereed to HSS publications, and not to natural sciences ones). Despite a small number of excessive APC charges, the average APC is moderate (1006€). A significant number of publications are available in gold open without any charges and in green open access without any embargo period.

Table 1 Gold/Green OA in H2020 yearly breakdown (source Background Information Note - Data on Open Access to Scientific Publications in Horizon 2020)

Complementary data on this report are available on the European Union Open Data Portal.