We propose that operational semantics may be used as an effective modelling tool even for large-scale object systems. In order to demonstrate this principle we introduce an expressive, yet semantically clean, core Java-like language, Java Jr. and provide it with an operational semantics which describes interaction across package boundaries using labelled transitions. We claim that the trace semantics generated by these labelled transitions is fully abstract for a natural notion of may testing for object systems and provide a detailed example to demonstrate the intuitive character of the semantic model.