This topic always draws out the most interesting responses no matter where you find it.
It can be messy. As a rule, we are not very good students of God's Word. We bring our own ways of reading and processing information to it, for one. We also have so many disparate voices insisting on being authoritative themselves and they are saying all sorts of different and often opposing or inconsistent things.
I like what Havok said about the OT and NT.
We should not view the revelation of God in the Word through two sets of glasses. The Bible is a long revelation of God, who is always the same. It is the revelation of God in His relationship with His creation. Because it is a practical revelation of an actual relationship, we are blessed to have concrete historical accounts of that revelation in real human time as opposed to a compilation of truisms and abstract ideas. One reason we fail to read it as a single, complex, continuing revelation of one God is because the faces, names and places do change as God's relationship outlasts one generation and is accounted for into the next, and so on.
It does little help to say that the OT is fulfilled in the NT. It is true, but it doesn't really mean anything in a practical sense to most readers.
The OT is not out of date. It is not "less authority" than the NT. There is no opposition between them. What the OT is, though, is the revelation of God, given to men by eyewitness accounts, by God's providential supervision of the recording of actual events, places and people, and by the agency of the Holy Spirit. The OT revelation is the history of God's specific revelation of Himself and His intentions for His creation on earth through one relationship with a multi-generational ethne with whom He bound Himself in three particular covenants: Abrahamic (through whose seed all nations will be blessed, the seed being Jesus), Mosaic (the Law of God as a tutor and master of the ways of God and the sin of man, designed to bring the particular line of Abraham through Issac and Jacob through time and by the guiding hand of God until the fulness of time and the person of a Jesus, son of Mary) and Davidic (a royal kingdom comprised of the line of Abraham, Issac and Jacob and ruled by descendants of David, culminating finally and forever in Jesus the Christ in the eschaton of all things.). The NT is the historical account of God's completion of His revelatory work and His redemptive work with His only begotten Son, Jesus, as the fullest and final revelation of God and as the redemption of the physical line of Abraham (Israel) and of the "nations" through faith in Jesus.
Without the OT, there is no NT. Without the NT, the OT is moot. Certainly, there are specific portions of the OT that do not have direct application to the nations, and the whole of the ritual law of the OT is made unnecessary by the sacrifice and resurrection of Jesus. Yet, these do not pit one of the testaments against the other, nor the covenants.
Finally, Jesus is the fulfillment of all things. It is before Him that every knee shall bow and every tongue confess that He is Lord. At the transfiguration, where Moses and Elijah appeared to Peter, James and John and spoke with Jesus, it was Peter's response to build three tabernacles including one for Moses. But, only a week before, Peter had said to Jesus, "You are the Christ, the son of the loving God." The Father Himself responded to Peter: "This is my Son. Hear ye Him," and Moses and Elijah were gone. There is no question that Jesus was, is and shall ever remain the One to whom we answer, the One to whom we must give account. He is the ahead of the Body and there is no other. There are no "degrees" of His authority, which is absolute and unique as the blessed and only potentate.