The EOSC faces as well technical, social and economic challenges on interoperability. These have to be overcome in order to provide an efficient EOSC architecture. Some solutions are in place in some domains, but these have to be expanded to the cross-disciplinary and/or international level. We will summarize what the results of our studies within the EOSCpilot on interoperability and FAIR data sharing are. Then we will discuss openly solutions, opportunities and showstoppers with respect to an interoperable and FAIR EOSC with key experts in the field.