Decision-making (DM) knowledge is often represented as a set of basic definitions such as alternatives, criteria, decision matrix, and decision itself. However, this domain is much richer, and many other related notions are useful to make right decisions: preferences, weights, thresholds, and so on. This knowledge must be formalized within a model. In addition, the practical needs for DM as well as the number of researches dealing with decision-making increasingly grow. We suggest using the ontology representation for representing DM knowledge. In this paper, we analyze different ontology fundamentals, we present the DM ontology (DMO) and its application to the selection on an ERP tool, and we show how the ontology fundamentals were used for representing DM knowledge.