The paper presents Ontology Definition Metamodel (ODM) that enables using Model Driven Architecture (MDA) standards in ontological engineering. Other similar metamodels are based on ontology representation languages, such as RDF(S), DAML+OIL, etc. However, none of these other solutions uses the recent W3C effort -The Web Ontology Language (OWL). In our approach, we firstly define the ODM place in the context of the MDA four-layer architecture and identify the main OWL concepts. Then, we define ODM using Meta-Object Facility (MOF). The relations between similar MOF and OWL concepts are discussed in order to show their differences (e.g. MOF or UML Class and OWL Class). The proposed ODM is a good starting point for defining an OWL-based UML profile that will enable using the well-known UML notation in ontological engineering more extensively.