Ong's notion that the writer's audience is always fictional is one of the most interesting and plausible ideas we have read so far, I believe. When a writer is creating their literary piece, it is impossible for him or her to see and adapt to whomever may read their literary work. Because literature circulates in such abundance, a writer has no way of altering and adapting their work accordingly because they can in no way predict who they should alter the work for. Like every piece of writing, and speech as Ong mentions, there is always an intended audience that the writer may be writing for (e.g. abused women, men of power, toddlers just learning to read, etc.) but they cannot expect that those are the only persons that will come across their literary work.
It is simply implausible to think that as a writer you are able to predict, and ultimately write for, one specific audience that will read you work. Ong states that "it would be fatuous to think that the writer addressing a so-called general audience tries to imagine his readers individually." I believe this opinion is one that can be most agreed with. As Barthes says, writing and literary works transcend far beyond the author's life so it is completely impossible that the author of a literary work could have the insight as to how their work will be read by people in different countries, and in their own countries, decades, even centuries, from the time he or she first wrote and published the work. Burke believes that the author of texts do not exist in their literary works, that they essentially "die" and their work is their remnants commenting on a particular time, sometimes fictional and other times not, in which the author's characters are situated in within the literary work. This ties into Ong's notion that all audiences are fictional because when an author comments on a particular time by writing it into a literary work, they must know that that work will be read by a large variety of audiences, both the intended audience or those familiar with the topic or issue being discussed, and those that are referencing the literary work for greater insight on the topic or issue being illustrated in the literary work.