Since we have God's written word, why so much emphasis on oral tradition?
You place a lot of emphasis on oral tradition in the Catholic Church, but doesn't the fact that we have a written Bible show this was the way we were intended to receive the word of God?
The preferred method of communicating the word of God was not in writing but by word of mouth. Much of the Old Testament was known orally for centuries before it was written down.
Jesus himself wrote none of the New Testament. He established a living Church founded on Peter and the apostles, and he told them to preach. We see in the epistles of Paul how anxious the apostle is about the welfare of the local churches he has established and how he wishes he could be there with them in person to guide and teach.
In 2 John 12 we see explicitly in the written word itself how the apostles preferred to communicate directly with their own lips: "Although I have much to write to you, I do not intend to use paper and ink. Instead, I hope to visit you and to speak, face to face."
The Bible is a testament to the oral tradition that was alive and already at work. Our source of the revealed word of God is Scripture plus Tradition--a Tradition that the Church Christ founded preserves and teaches. Much of that Tradition was reduced to inspired writing under the influence of the Holy Spirit.