Today, mass presence of distributed energy resources (DERs) connected to the grid is often seen as having adverse effects on grid reliability and robustness. The apprehension is that it complicates or even compromises network management by distribution system operators (DSOs). The central aim of the Open System for Energy Services (OS4ES)  project is to provide a solution that closes the current information, communication and cooperation gap between DERs and DSOs. To this end, the OS4ES project delivers an innovative Open Service System that enables dynamic DER-DSO cooperation and has future potential for new businesses. A Distributed Registry for DER systems offers involved actors the opportunity to reserve the aggregated flexibility of DER systems (even forming dynamic Virtual Power Plants) as a grid management service in order to improve Smart Grid robustness and reliability. OS4ES will be based on standardized and interoperable communication interfaces, as well as generic interfaces among components producing, consuming or storing electrical energy. © 2015 IEEE.