Although the necessity of an ontology and ontological engineering is well-understood, there has been few success stories about ontology construction and its deployment to date. This paper presents an activity of ontology construction and its deployment in an interface system for an oil-refinery plant operation which has been done under the umbrella of Human-Media Project for four years. It also describes the reasons why we need an ontology, what ontology we built, what environment we used for building the ontology and how the ontology is used in the system. The interface has been developed intended to establish a sophisticated technology for advanced interface for plant operators and consists of several agents. The system has been implemented and preliminary evaluation has been done successfully.