Re-engineering of successful pre-OWL ontologies or other formal ER or UML system models towards OWL DL compliance opens new possibilities in ontology debugging, enabled by the formal semantics and automated reasoners developed for OWL DL. Meanwhile the transformation of legacy ontologies to OWL DL is a challenging and interesting task, which we illustrate in this paper on the example of OntoSem — a lexical common-sense ontology and an application framework for deep semantic analysis of natural language texts.