The oral word is more important to illiterate people, which included Mohammad and nearly all of the early converts. The written word becomes more important when people learn how to read.
Mohammad, who spoke only Arabic, said that the Koran could only be understood in the original Arabic. This is a major limitation of Islam, of course. Arabic is not a particularly easy language to learn for those who speak languages from non-Semitic language groups
There are Muslims who say that Mohammad's illiteracy PROVES that the Koran is a miracle because somehow Mohammad, and illiterate, managed to memorize the whole thing, and its prose is so berautiful that it could never have been composed by a mere mortal.
I tend to doubt both propositions. I have read rather a lot of two translations of the Koran, and it is disjointed, vague and filled with endless repetition of set phrases. It is more interesting than Leviticus or Deuteronomy, but pretty boring when compared with Exodus.