Abstract. Though ontologies are widely used to solve some specific interoperability problems, there is no specific ontology defining what interoperability actually is, independently from any domain. In this paper, we propose and discuss a first version of such an ontology, namely the OoI (Ontology of Interoperability), which we formalized using the Ontology Web Language (OWL). On the basis of previous research efforts having lead to UML formalization of our model of Interoperability, we use this paper for presenting the OWL version and for linking and comparing it with other models dealing with Interoperability: maturity models for interoperability like e.g. the Levels of Information System Interoperability (LISI) model, and the Model Morphisms ontology (MoMo), which deals with interoperability of models. Finally, we illustrate in a brief use case how the OoI could be used with MoMo to provide solutions to interoperability problems between two models.