Ontology has already a rather long history in the computer science. It helps to represent knowledge in the domain of interest and make it available both for humans and machines. For many years, ontologies were mostly considered as an object for academic research, but nowadays they are getting used in a growing number of applications. The suggested paper provides a brief overview of practical ontology implementation in decision support systems. Typical knowledge-intensive tasks were used to organize the overview: one example of a system was provided for each task.