Compared to the general population, people with OCD are 10 times more likely to have a bipolar disorder (BPD) diagnosis. Research has shown that BPD comorbidity is associated with more severe OCD, and worse health and quality of life outcomes. Treatment for people with both conditions also differs from standard OCD treatments to account for BPD symptoms. As a first diagnosis of OCD is associated with a higher risk of a later diagnosis of BPD, understanding the causes behind co-occurring OCD and BPD is necessary. These causes may be shared genetic and/or environmental risk factors that increase the chance of both conditions occurring, and/or causal relationships where one disorder increases the likelihood of the other. Using a large national demographic and clinical dataset and large-scale genetic studies, this study aims to explore shared genetic and environmental risk between OCD and BPD, and assess whether OCD increases the likelihood of BPD. With these findings, the study will develop models to predict a future BPD diagnosis in people with OCD.