No, Discworld wasn't the first source to use "ook" for monkey sounds.
It's not a source I would have expected at all, but "ook" is used in the 1976 scientific publication Communication Mechanisms and Social Integration in the Black Spider Monkey, Ateles fusciceps robustus, and Related Species:
The ook-ook vocalization, although associated with grappling, is produced with many subvariants and may grade into aggressive growling or be produced in a low intensity variant, which may accompany certain aspects of sexual behavior. At no time was it inferred that ook-ook and squeak vocalizations were entirely sexually motivated or occurred entirely in a sexual context.
It was also used in What a Spot!: A Musical Farce in Three Acts (1975; a far more expected source):
(Lolita, a friendly female ape, comes loping out of underbrush, flings herself up in amiable fashion onto Rob from behind.) [...]
LOLITA. (Makes sounds to the effect of:) Ook-ook! Eeek-aak-eek!
