Open Access repositories, whose number has been steadily rising, are an important component of the global e-Research infrastructure. The real value of repositories lies in the potential to interconnect them to create a network of repositories, a network that can provide unified access to research outputs and be (re-) used by machines and researchers. However, in order to achieve this potential, we need interoperability. The purpose of this paper is to provide a high-level overview of interoperability of Open Access repositories, identify the major issues and challenges that need to be addressed, stimulate the engagement of the repository community and launch a process that will lead to the establishment of a COAR roadmap for repository interoperability.