Many IRIS GL programs use the built-in windowing interface provided by IRIS GL. In contrast, OpenGL relies on X for all its windowing functionality. If your application uses IRIS GL functions like winopen(), your windowing code needs to be rewritten for X. See the OpenGL Porting Guide for more information.
Note that the term "mixed-model program" is no longer relevant when you work with OpenGL, since all OpenGL programs use the native window system for display and event handling. (The OpenGL API, unlike IRIS GL, has no windowing calls).