Many researchers agree that the reuse of ontological components is an important research area in Knowledge Acquisition. However, it has been argued that there is still much to do in this field. For example, one of the topics that requires more research is that of developing methods to build ontologies. In this sense, it is important to define formal methods which provide us with operative frameworks to build ontologies. In this paper, for problems that satisfy a well-defined set of assumptions, we propose a mathematical approach that permits to build domain knowledge ontologies. We present such an operative framework based on both Sets Theory and mereological considerations. The approach comprises a set of ontological operators to extract domain knowledge. Finally, an example is put forward showing the application of these ontological operators.