Is the Bible the supreme authority?

 No. We do not believe the Bible is a q'aran - the verbatim word of God. It is a collection of manuscripts written and compiled by inspired but fallible man.

For some 300 years the early church didn't even have a 'bible', that is, the New Testament gospels as we have today in one book. Writings were scattered. Oral tradition was how the gospel of Christ was primarily communicated. The theology and practices predate the writings. The New Testament writings are meant to support and accompany the existing theology of the early Christians. Without the apostolic tradition, the scriptures cannot be taken alone and interpreted independently, without big problems (see all the branches of Christianity that arose from the 'sola scriptura').

The Orthodox Church could still exist without the writings, because the theology and traditions predate the writings and go back all the way to Christ himself. This is why you can’t read the gospels independently and derive your own interpretations, assumptions and theology. It was a companion to the existing teachings of the apostles and Christ himself. As such, the gospels can only be properly interpreted with the full understanding of the existing teachings and traditions passed down through the apostles and saints. The scriptures serve to affirm and reinforce what was taught, lived and practiced by the early church. But without connection to the early church (which is the Orthodox church), the writings lack a vital dimension. 

The Orthodox Church is definitely not 'sola scriptura', or scripture alone. If you take scripture alone and try and interpret it on your own steam, you get thousands of different branches e.g. Jehova's Witnesses. Mormonism. Cults. It's not a good idea. The most fundamental reason why the Church fragmented into so many branches after the first 1000 years of Christian history was precisely because people decided to shirk apostolic tradition and interpret for themselves.