This paper presents a model for object-oriented operations in which operations consist of two parts: an internal state transition and an external interaction with other objects. We first present a simple version of the two part model, and explain howthis model has been used to develop a strictlymodular, compositional semantics for a simple sub-language of Object-Z. Differences between the operation calculii of the simple sub-language and of Object-Z are identified. We extend the simple model to accommodate distributed operators, and then define a syntactic translation from “full” Object-Z to the simple sub-language. This shows that the two part model is sufficiently expressive to describe many notions about operation invocation in object orientation.