On page 11, Ong shares that writers must fictionalize an audience since no one is actually there to contribute feedback as they "listen". The author must think of an audience as a general whole, the real social, economic, and psychological state of possible readers. If one is unable to create an intended audience they should use the voice of past authors. The example Ong uses for this idea is Samuel Clemens' Tom Sawyer. The road for an audience is two-sided: first the writer must construct in his imagination an audience cast in some sort of role. Secondly, the audience must correspondingly fictionalize itself. The reader must play the role in which the author cast him, which seldom corresponds with his role in real life (p. 12).
I personally believe that casting an audience is extremely important before writing or speaking a piece. Knowing the audience you are attempting to reach will help you formulate your content in the best way possible for the reader/listener to grasp. I do no believe that the audience changes from written to oral text, just the way in which the text is presented. As an author/orator it is important to know the audience and to adjust accordingly.