Google Books lists the book "The behavior of Ateles geoffroyi and related species" by John Frederick Eisenberg, Robert E. Kuehn, published in 1966, as including the paragraph:
Auditory communication by [the spider monkey] appears to be well-established for some sounds, especially the whinny, ook ook, squeak, and the high-intensity agonistic sounds such as the roar and cough.
The construction of the sentence suggests that the reader would be familiar with the various descriptions, including what the difference might be between "whinny" and "oook ook". At the very least, it indicates that "ook ook" was used as the description of the vocalisation of at least one type of monkey prior to Terry Pratchett's Discworld novels.
It might be of interest that I decided to search Google Books for ook ook, rather than simply ook, in part because it seems to me to be more descriptive of a monkey's sound, but also because searching for ook on it's own picks up typographical errors from words like look.