The Web Ontology Language OWL is a prominent ontology language for specifying ontologies. Although OWL ontologies are well-used for representing and reasoning about knowledge in various domains, they are sparsely studied for visual language specification. The work in this paper, therefore, explores OWL for visual language specification by specifying the concrete syntax of selected UML class diagram notations in an ontology. The selected diagram notations are specified as spatial configurations of primitive elements and qualitative base spatial relationships of Region Connection Calculus-8 (RCC-8). Furthermore, the automated reasoning features of ontology reasoners are investigated to verify the completeness and the correctness of the specification. The verification results indicate that the given specification needs to be revised to support applications to draw the selected notations. The value of such a specification in supporting a semantic diagram interpretation application is demonstrated using the automated instance classification feature of ontology reasoners.