The objective of this paper is to describe by a Platform Independent Model, the formal specification of an ontology-based service for electronic health records interoperability.The GCM is used as a framework for the service's architectural design. The formal specification of the service is an extension of the OMG CTS 2 specification. A review of mapping approaches is also provided.The paper describes the service' information and computation models, including the mapping process workflow. The platform specific implementation (Platform Specific Model) is provided as a set of WSDL interfaces. The specification includes ontology mapping algorithms and tools needed.