Based on every explanation of "Oneness" teaching as it concerns the Incarnation this is how it has seemed to me:
Jesus the man was a veil, or robe, or tent in which God dwelt while on earth. The flesh of Christ covers, or veils upon the Divinity of God (the Father) who dwelt therein.
Jesus, thus, is little more than God wearing a human suit for a time, in order to accomplish the task of atonement.
This in stark contrast to the orthodox teaching:
Jesus is the God-Man. He is not God veiled behind human flesh, God dwelling within a human being who functions as the outer shell of the Divinity. No. Jesus is true God and true man. The true joining of Deity and humanity together in the unity of His Person without separating the natures and without confusing them.
When Jesus prays, "Our Father..." this is God praying to God. Son to Father. The Son is God. The Oneness teaching is that the Son refers to the humanity worn by God; the orthodox teaching is that the Son is Himself true God, and that there is therefore a real and actual relationship between Father and Son.
The Oneness teaching preaches a three-faced God, a God of three masks; Father above, Son in the flesh, and Holy Spirit dwelling in us.
In stark contrast the orthodox teaching is that the One God is Holy Trinity. There is real Father, real Son, and real Holy Spirit; and the Three are One, eternal, consubstantial, uncreated, Almighty.
The Oneness view of Christ ultimately destroys the reality of the Incarnation. It isn't God inhabiting a man, God veiled by flesh; it is the union of God and man together in the Person of Jesus Christ. This is why we confess that He is True God, being eternal and only-begotten of the Father; and He is True Man, being conceived in and born of the Virgin Mary.
Without separation, without division, without confusion, without confounding. One Christ, Lord, Son, God, Word, Man, Savior. Who from eternity past is God with God and God from God, True Son of the Father; who from His conception is True Son of Man, Child of Mary, and is from now and everlasting Risen Lord and Christ, seated at the right hand of the Father, from which He will come again in glory to judge the quick and the dead, King of kingdom everlasting, God-Man, Jesus Christ our Lord and Sovereign.
-CryptoLutheran