This paper evaluates the performances of the OWL 2 reasoners Her-miT, FaCT++ and TReasoner in the context of an ontological decision support system in designing entity-relationship diagrams during software development. First, I described a developed ontology which is the knowledge base of the de-veloped application for designing databases. In the first set of experiments I compared how the classification and realization time of the DBOM ontology varied when increasing the ABox with ERD elements individuals. In the second set of experiments the consistency checking time of the DBOM ontology was estimated by increasing the ABox with ERD elements individuals.