(OOA) Defining all of the types of objects that do the work in a system and showing what user interactions are required to complete tasks.

This phase concerns with determining the system requirements and to understand the system requirements build a use-case model. A use-case is a scenario to describe the interaction between user and computer system. This model represents the user needs or user view of system.

It also includes identifying the classes and their relationships to the other classes in the problem domain, that make up an application.