This paper presents an approach for visually modeling OWL DL and OWL Full ontologies based on the well-established visual modeling language UML. We discuss a metamodel for OWL based on the Meta-Object Facility, an associated UML profile as visual syntax, and transformations between both. The work we present supports model-driven development of OWL ontologies and is currently undergoing the standardization process of the Object Management Group. After describing our approach, we present the implementation of our approach and an example, showing how the metamodel and UML profile can be used to improve developing Semantic Web applications.