Bipolar disorder (BD) is a serious psychiatric condition of unknown etiology but thought to result from a combination of dysfunction of brain biology, on a background of genetic, social, and personal contributing factors. The complex etiology and often diverse clinical manifestations of bipolar disorder pose challenges in establishing a clinical diagnosis. To integrate the large amounts of heterogeneous illness susceptibility knowledge, data, and metadata from multiple sources, we propose the development of an interoperable and integrative ontology of bipolar disorder (OBD) as a systematic framework to combine and represent knowledge from diverse sources. The advantage of OBD is the capacity to reuse, integrate, align, link, and structure terms from multiple ontologies under the upper-level Basic Formal Ontology. In use cases, we demonstrate how an OBD can be applied to map out the genetic predispositions and clinical symptoms of BD, aimed towards facilitating improved accuracy in diagnoses and personalized treatment plans. The OBD supports data FAIRness (e.g., findability, accessibility, interoperability, and reusability)and encourages collaborative and multidisciplinary studies of BD using diverse domains and ontologies.