Reasonable embargoes of 6 months (STEM) and 12 months (SSH) are allowed if necessary. 

Each beneficiary from an Horizon 2020 grant must deposit the work in a repository as soon as possible and at the latest on publication.
Each beneficiary must ensure Open Access to the deposited publication — via the repository — at the latest:

Related Documents:

Fact Sheet: Open Access in Horizon 2020
Fact Sheet for Project Officers in Horizon 2020
Fact Sheet for Project Coordinators in Horizon 2020
Guidelines on Open Access to Scientific Publications and Research Data in Horizon 2020
Multi-beneficiary General Model Grant Agreement