Sharing information and knowledge among heterogeneous health information systems requires semantic interoperability. Most integration projects address semantic interoperability by implementing HL7 version 3 standard interfaces. However, it is challenging to achieve computable semantic interoperability with HL7 because of i) the complexity of the standard, requiring HL7 experts in the interface implementation process ii) inconsistencies and overlapping of the different HL7 information models (RIM, D-MIMs, R-MIMs, C-METs), and iii) instability of the different HL7 version 3 models. In this paper, an ontology-based service for health systems semantic interoperability is proposed. This service includes three main components: i) the conceptual model formalization component, responsible to represent the conceptual information models of the applications to be integrated as formal application ontologies; ii) the ontology mapper component; responsible to realize the semantic mapping between the formal application ontologies using a domain ontology, therefore solving inconsistencies found in the source application ontologies; (iii) the automatic interface generator, responsible to create and to maintain HL7 version 3 interfaces. The service presented in this paper is primary focused on the implementation of HL7 interfaces to integrate legacy systems. However being supported in an ontology-based mapping of HL7 information models, it can also support semantic interoperability among healthcare services and applications.