I need to clarify my question.
What does “the oral transmission of God’s word” mean?
You say this: “there was a long time where oral transmission of God’s word was there but eventually it was written down”.
I am not sure how that relates to this topic.
Well, considering some Catholics seem to believe that simply speaking something translates into “tradition”, it does relate.
oral transmission is not by default automatically tradition.
**What does “the oral transmission of God’s word” mean?**
This is what Holy Tradition is all about.
There were no books or Bibles at that time and people had a great talent for memorizing.
Thus, we have the end of John’s Gospel, where he states that not everything said by Jesus can be written down — consider the lack of Papyrus to make scrolls, for example.
When we talk about the New Testament Scripture, for example, we know that there was at least a decade or two between when Jesus ascended up to heaven and the first "gospel" books were written down. The teachings that Jesus relayed while He was here were passed down verbally/orally through believers to those who wanted to know what Jesus said. Gradually, the entire New Testament was completed with the writings of the Apostles and their disciples who wrote as they were led along by the Holy Spirit.
The same thing happened in the time before Christ. The history of creation, the establishment of the Jewish nation through Abraham, Isaac and Jacob - all was passed along by oral tradition (tradition is "teachings") until the time Moses was tasked with writing it down in the Pentateuch (the first five books of the OT). There were writings from the prophets of the Lord as He directed them, as well.
Catholicism, as well as some others, teach that there was much revealed truth that was passed down orally that never did get written down. They call this "Sacred Tradition". The problem is that they don't really have any proof that what they say was orally passed down from the Apostles ever actually came FROM an Apostle. I hold to the contention that the Holy Spirit ensured that everything we needed to know for our salvation, our walk in the faith, our responsibilities towards God and each other, and His plans for our world WAS written down in Sacred Scripture. Because it is the Divinely-inspired word of God, it stands as THE authority over anything man has devised. The church is not an authority over Scripture but is its servant expected to be obedient to what God tells us.