Psychological diagnosis is the process by which mental health professionals determine if problems that affect a person meet all the specific criteria for a psychological disorder. In recent years, decision support systems (DSS) have helped practitioners in the field of psychological clinical diagnosis with notable results. Given that ontologies are created, among other goals, to allow different sorts of formal reasoning, they are seen as valid artefacts to support a new generation of DSS for mental health professionals. Although some initiatives have emerged in order to build realistic ontologies of mental diseases, this field presents a remarkable heterogeneity of data and two different clinical classification systems, which is why this paper presents an ontology with the aim of reusing existing works and serving as the key element of a clinical DSS in the field of mental disorders.