What is the basic Protestant understanding of oneness in these Scriptures?

There seems to be no agreement on just what oneness means.

John 17:20-21 Neither pray I for these alone, but for them also which shall believe on me through their word; That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us: that the world may believe that thou hast sent me.

It is Jesus stated desire that the disciples and future Christians are joined with him in the same unity he has with the Father. We can get a picture of this unity from other verses;

John 10:30 I and my Father are one.

John 8:29 And he that sent me is with me: the Father hath not left me alone; for I do always those things that please him.

John 5:30 I can of mine own self do nothing: as I hear, I judge: and my judgment is just; because I seek not mine own will, but the will of the Father which hath sent me.

John 14:9-10 Jesus saith unto him, Have I been so long time with you, and yet hast thou not known me, Philip? he that hath seen me hath seen the Father; and how sayest thou then, Shew us the Father? Believest thou not that I am in the Father, and the Father in me? the words that I speak unto you I speak not of myself: but the Father that dwelleth in me, he doeth the works.

We can see from the example of Jesus and the Father that there is no selfishness or self-interest to impede unity. This is more than a unity based on proximity, this is a true unity that comes from complete surrender of self and total dependence on God.

We can see from the description of how the church should function that the objective is for us to obtain this unity;

Ephesians 4:11-15 And he gave some, apostles; and some, prophets; and some, evangelists; and some, pastors and teachers; For the perfecting of the saints, for the work of the ministry, for the edifying of the body of Christ: Till we all come in the unity of the faith, and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fulness of Christ: That we henceforth be no more children, tossed to and fro, and carried about with every wind of doctrine, by the sleight of men, and cunning craftiness, whereby they lie in wait to deceive; But speaking the truth in love, may grow up into him in all things, which is the head, even Christ:

When we become Christian, we have the Spirit of God give us new life.

Romans 8:2 For the law of the Spirit of life in Christ Jesus hath made me free from the law of sin and death.

This is just as significant as our physical birth

1 Corinthians 15:45 And so it is written, The first man Adam was made a living soul; the last Adam was made a quickening spirit.

This new life entails having all three persons of the Trinity living inside of us working to make us more like Jesus.

John 14:23 Jesus answered and said unto him, If a man love me, he will keep my words: and my Father will love him, and we will come unto him, and make our abode with him.

John 14:26 But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you.

This perspective on oneness and unity may seem alien and even supernatural to most of us. This is because this unity requires our participation to achieve.

1 Thessalonians 5:19 Quench not the Spirit.

The process of becoming like Jesus has come to be seen as an almost mythical "deeper" Christian life disconnected from the "real" world. It seems almost at every turn there is someone who wants to turn Christianity into an experience, a system of rules to follow, or a political movement. The idea that the selflessness of love (described in 1 Corinthians 13) is actually achievable is made to seem unrealistic by Satan to controls the course of the world (for now and insofar as he is allowed by the Father).

Ephesians 2:2 Wherein in time past ye walked according to the course of this world, according to the prince of the power of the air, the spirit that now worketh in the children of disobedience:

The unity that Jesus describes in John 17 is real and does not need to be explained away as an allegory for a future home. Jesus gives his relationship with the Father as the illustration of the unity he desires for us with him. The fact that this is so difficult to consider is not an indication of some hidden meaning that we need to yet uncover, but that we have come to accept a Christian life so devoid of vitality and enmeshed with the world that the expected Christian life seems alien and unobtainable.