Background This nationwide register‐based study investigates how often obsessive–compulsive disorders (OCD) with different age at diagnosis occur in affected families compared to control families. Furthermore, the study addresses the impact of certain risk factors, that is, sex, degree of urbanization, year of birth, and maternal and paternal age at birth. Methods A total of N = 2,057 child and adolescent psychiatric subjects born between 1952 and 2000 and registered in the Danish Psychiatric Central Research Register developed OCD before the age of 18. In addition, N = 6,055 controls without any psychiatric diagnosis before age 18 and matched for age, sex, and residential region were included. Psychiatric diagnoses were also obtained for the first‐degree relatives as a part of the Danish Three‐Generation Study. A family load component was obtained by using various mixed regression models. Results OCD occurred significantly more often in case than in control families. Having a mother, father, sibling, or an offspring with the disorder was proven to be a risk factor. Maternal age above 35 years, male sex by tendency, and ascending year of birth were associated with having OCD. Furthermore, case relatives did not develop OCD earlier than control relatives. The risk of OCD in the case probands was significantly increased when first‐degree family members had either OCD, or tic disorders, or affective disorders, or anxiety disorders. Conclusions These findings based on a very large and representative dataset provide further and very solid evidence for the high family aggregation of OCD.