Realization of Semantic Web requires structuring of web data using domain ontologies. Most data intensive websites are powered by relational databases whose design process involves developing conceptual model using E/R or Extended E/R diagrams. This paper discusses the implementation details of a tool that builds domain ontologies in OWL (Ontology Web Language) from Extended E/R diagrams. Ontology development being a knowledge intensive task, our tool would be helpful in reducing the developmental efforts by automating the process. We bring out the differences and the similarities between the expressive capabilities of the two conceptual modeling methods, namely OWL and Extended E/R diagrams. Copyright 2005 ACM.