In this paper is proposed an ontology based on the object-oriented paradigm for the development of explanatory systems to simulate, in an appropriate way, the professor-student interactions carried out in the teaching processes. The knowledge explained in the dialogue process is represented at different levels of complexity by means of a network of multi-connected units of knowledge. Under the proposed model an open explanatory dialog system is built and incorporated into the Virtual Chemistry Laboratory, a virtual system for practical chemistry learning, developed by the authors.