In the spirit of interdependence of the different areas of CHI research, we present a description of OPOSSUM, a visualization tool inspired by concepts from heterogeneous databases. OPOSSUM is a tool for visualizing and editing structured data; we use it to view and modify objectoriented database schemas. OPOSSUM is based on a formalism that allows declarative descriptions of the following: 1) a model describing the schema to be visualized, 2) a model describing visualizations, and 3) a mapping between the two models. The formal approach makes OPOSSUM very flexible, and promises solutions to several problems of schema visualization.