Ontologies have been used in safety engineering for the representation of knowledge in the form of concepts, facts, and procedures. In this work, an ontology for Hazard and Operability (HAZOP), risk assessment, and related process and plant concepts, is proposed. It is intended to support participants during HAZOP studies. The ontology is designed based on competency questions which can be answered using ontology queries. The developed ontology is applied within a case study to provide knowledge to fill a HAZOP worksheet, including the risk assessment, of a gasoline storage tank system. The results demonstrate that the ontology is well-suited to answer the formulated competency questions correctly. Additionally, metrics have been used to assess the quality of the ontology and to be able to compare ontologies on a quantitative basis. Within future research, the proposed ontology could be enhanced and integrated with an upper ontology.