Some people with obsessive–compulsive disorder (OCD) experience recurrent intrusive sexual, aggressive or death-related thoughts and as a result may be subjected to lengthy or inappropriate risk assessments. These apparent ‘primary’ risks can be dealt with relatively easily through a careful understanding of the disorder's phenomenology. However, there are other, less obvious ‘secondary’ risks, which require more careful consideration. This article discusses the differentiation of intrusive thoughts and urges in people with OCD from those experienced by sexual or violent offenders; assessing the risk of self-harm and suicide; discussing the nature of repugnant obsessions with a patient; assessing risk of harm and violence to the dependents and family living with someone with the disorder; and assessing the lack of insight and the use of the Mental Health Act. Issues specifically related to children and young people with OCD are also highlighted.