Decision support is one of the main objectives of ontology-based knowledge management systems. However, there is no standard method that would define how to model decisions in ontologies. Despite many research efforts and established methods for decision modelling and support, they have not yet been systematically applied to the field of ontologies. This paper proposes an ontology based multi-criteria decision making method that enables one to define decision models using ontology as the base construct. It structures decision models in such a way that the problem solution can be obtained by reasoning upon the ontology. We propose a generic approach that can be applied to an arbitrary domain. The proposed method is based on qualitative multi-criteria decision making, which is applied to the field of ontologies. OWL is used as the ontology representation language. As a proof of concept we have developed an ontology-based decision support system for an electric power transmission company. The proposed method represents an important step forward in the field of ontologies. Its main advantages are: a higher level of decision support provided by the ontology, direct use of information captured in ontology for decision making, a higher level of business process automation, and reuse of decision model concepts in definitions of more complex ontology concepts.