Our software platform is composed of several tools implemented as a set of Eclipse plugins using the Eclipse RCP standard "extension point" mechanism. This is the lightweight software integration model we decided to use during the conception of the OPSWP (see deliverable D2.2 for details), whose graphical consistency is guaranteed a minima thanks to the basic UI mechanisms (SWT widgets, workspace, etc.) of the Eclipse RCP. Each one of the integrated software tools is part of the model-driven workflow used by our platform to implement its features; however, these tools are not basically conceived to work together, thus undermining the coherency and usability of the OPSWP. Since the usability of our future software platform is a major concern for us, we are putting much effort into the harmonization of the various software elements to offer the best possible user experience to the system architects who will be our main users. The purpose of this document is to describe how we intend to provide good ergonomics to the OPSWP: we will especially explain the choices we made about the GUI conception. Ergonomics are only a question of usability, which depends on the good organization of GUI, on good use of visual components, forms, and so on. Ergonomics don't define neither the graphic charter nor the visual identity of our software: these "artistic" considerations are part of the project's task 6 (dissemination). The present study is divided into several consecutive parts: we first begin by analyzing user profiles and their related tasks, then we review the technical and architectural constraints that are imposed to us. From all of these, we eventually build a style guide, that defines the usability requirements that shall be followed as guidelines to implement the graphical user interfaces (GUIs) of the OPSWP software elements with the expected consistency. This document - not planned at the beginning - is part of task 2 of the OP project. This task is under the responsibility of the INRIA NGE team, and its main goal is the specification, conception and implementation of the OPSWP.