The OSA model has three main sub-models. The interaction model describes the origin and destination of events as well as the source and destination of information transfer. There are many ways to approach analysis. With real-time systems we often start by identifying all of the external events a system must respond to. In OSA these events are represented by interactions.
Interactions show the source and destination of events and information.
Data flow diagrams have several problems, however. First, they are not object-oriented. They show only the interaction between processes and not between objects or object classes. Next, they show only data flow. This resulted in a bifurcated model that never mixed event and information flow.