Ishikawa diagrams, also known as fishbone or cause-and-effect diagrams, are a widely known visual tool for performing root cause analysis (RCA). Although Ishikawa diagrams originated in the manufacturing sector, the tool is also actively used in other areas such as healthcare or business due to its simple structure, which requires little or no training beforehand. Though Ishikawa diagrams are valuable sources of knowledge, they lack rich semantics to effectively process them. As a result, knowledge engineers tend to ignore Ishikawa diagrams and choose other means to collect knowledge, although domain experts are familiar with the RCA tool and it is highly accepted. This paper presents the Ishikawa diagram ontology which enables the explicit modeling of Ishikawa diagrams as visual artifacts, their encoded knowledge and the process of their creation by reusing and extending existing ontologies. The ontology was developed using the LOT methodology. We have created a dataset of Ishikawa diagrams and describe a fictional use case to illustrate the intended use of the presented ontology.