The emerging EOSC ecosystem will unquestionably require a minimum set of interoperability metadata to ensure basic discoverability, navigation and accessibility between systems. OpenAIRE-Advance is collaborating with the projects EOSC-hub and EOSCpilot to ensure metadata interoperability and service integration between three infrastructures that are important building blocks of the EOSC: OpenAIRE, EUDAT and EGI. Different types of collaborations are envisaged, ranging from service and data integration to joint activities for the definition and implementation of interoperability guidelines. Specifically, OpenAIRE-Advance: (i) runs pilots for the integration of two EOSC-hub services as OpenAIRE content providers: EUDAT B2Share and EGI Virtual Appliance Database; (ii) collaborates with EOSC-hub and EOSCpilot for the design, development and maintenance of the EOSC portal and catalogue; and (iii) collaborates with EOSCpilot for the definition of Open Science indicators useful to generate reports on different aspects and facets of Open Science.