One of the major misreadings of Ong's work, I think, is the belief that he believes something just magically happened, that somehow a switch in the human brain was flipped from "oral" to "literate." The fact that much of Ong's theories rely upon an auditory->visual shift which is itself tied to conceptions of space. Even this isn't a complete picture as I haven't referred to presence and a number of other things. Any way, I found the following passage interesting, not because any of it is new to me but because of how it works as a synopsis. It's from "From Allegory to Diagram in the Renaissance Mind: A Study of the Significance of the Allegorical Tableau." The Journal of Aesthetics and Art Criticism 17.4 (1959): 423-440.
"Printing itself is a kind of captial phenomenon in the concentration of thought on spatial forms. With the invention of printing from movable type made from matrices struck with a punch--the essence of fifteenth-century typographical developments--meaning was committed to space or 'stored' in space in a more definitive way than ever before [my emphasis]. Picture writing had been the initial commitment of the word to space. The alphabet had gone further, breaking down the word, a denizen of the world of sound, into spatially discrete parts. But in a manuscript culture these parts or letters had had to be 'manufactured' by a scribe as they were used. They pre-existed only in his imagination. With the invention of printing from movable type, the parts or letters, and even the parts from which the letters are made (matrices and punches), are prefabricated. As maneuverable parts, the printing types are actually stored in space, in little compartments in a case, from which they are moved onto a composing stick, thence onto a stone, and finally onto a press, where multiplication is effected by mere local motion bringing paper into contact under pressure with the printing form.
"Moreover, the books which finally evolve from this process are quite different from manuscript codices with regard to the relationships of the words or 'contents' (this spatial notion of 'contents' actually comes into currency only after printing is developed). Now, for the first time, a schoolmaster can say to his class, 'Everyone turn to page 7, and in line 4 from the top of the page look at the third word from the left.' For every book in the class will have the words locked into position in exactly the same place on each page--a condition which did not obtain in a manuscript culture, where the same words were found on quite different pages and in quite different positions on the page in the various manuscript copies of a work, so that the auditory memory and not the visual tended to be the primary operative tool. But with typography, the ability to find and deal with some bit of knowledge tends to be more an operation in space than in oral mnemonics. The bright student is now rather more likely to be visile rather than audile" (435-36).
To further illustrate the importance of auditory->visual shift and spatial understanding, here's a passage from "Ramus and the Transit to the Modern Mind," originally published in Modern Schoolman 32.4 (1955): 301-11, and quoted here from An Ong Reader, 229-238.
"The study of Ramism makes it evident that to understand the history of method we have to abandon our own favorite lines of explanation and get back to the issues as they really existed. The basic issue was not the struggle between inductive and deductive method, for there never was any serious or concerted opposition to inductive method but, if anything, too much respect for it--philosophers commonly took it for granted that induction was essential groundwork and therefore that it was easy to do and needed no special attention. The basic issue was the struggle between sound and sight, between habits of thinking based on listening to voices and habits of thinking based on looking at surfaces, between living in a world inhabited by persons who talk back and living in a world occupied by passive objects scattered in 'systems' through the new Copernican space. The real obstacle in the way of fuller inductive development was not deduction but the voice and person of the teacher, who kept talking all through the scholastic centuries. The way in which teaching actually blocked observation in dissection as practiced in medical lectures has been shown by Herbert Butterfield [The Origins of Modern Science. London: G. Bell and Sons, 1950] (32-33). This situation is symptomatic of the whole state of mind at the time. But this same teacher proved all-important, nevertheless, in paving the way for a more inductive approach, since his incessant talking helped reduce the dialogue of dialectic to a monologue and thus was a preparation for the more complete elimination of personal components in the scientific situation in favor of an 'objective' apersonal approach" (236).
"This drive toward the spatial, this reinforcement of the visile component of cognition, is a drive toward the construction of the observational, depersonalized collection of objects in terms of which we picture the world today, because it is a drive to think of things as surfaces, objects, rather than as symbols or as persons with voices. But the drive in Ramus' case is completely blind: he has no noteworthy expressed partiality for an observational approach at all. What he wants is 'arts,' something to know that is clear, distinct, set down once and for all in a book, and in the last analysis, picturable--the visile Ramus is the forerunner of the visile Descartes here. At this point, the way is prepared for 'subjectivity' by the death of the element of dialogue in dialectic. The two-part Socratic personal interchange is gone, and even the monologue of the teacher is gone--in other words, persons and voice are gone. An art is now a 'thing,' not a possession of the mind but something with surface, like the rest of the coming Newtonian mind" (237).