When Knowledge Graphs (KG) are used in practice, in particular for complex objects modeling, Semantic web recommends building KGs using existing common and domain ontologies as the base ontologies for modeling and extend them when developing new applications. In this work, detailed ontology for telecommunication networks (TN) monitoring system based on KG is discussed. The proposed ontology uses common ontologies and TN domain ontologies like Telecommunication Services Domain Ontology (TSDO) and TOUCAN Ontology (ToCo) as basic ontologies. To build the required ontology TSDO is extended on application level, in particular, new sub-ontologies for subdomains tasks solving are added. The case study of creation an ontology for TN monitoring system of a Cable TV operator is introduced. The developed ontology is described using Web Ontology Language (OWL).