The Ontology of Organizational Structures of Trauma centers and Trauma systems (OOSTT) is a representation of the components of trauma centers and trauma systems coded in Web Ontology Language (OWL2). It is developed collaboratively by domain and ontology experts in the NIH-funded CAFE (Comparative Assessment Framework for Environments of trauma care) project (1R01GM111324-01A1). It will be used as the ontology backbone of a graphical user interface comparing graphical representations of organizational structures.