John 10:30 I and my Father are one. 31 Then the Jews took up stones again to stone him. 32 Jesus answered them, Many good works have I shewed you from my Father; for which of those works do ye stone me? 33 The Jews answered him, saying, For a good work we stone thee not; but for blasphemy; and because that thou, being a man, makest thyself God. 34 Jesus answered them, Is it not written in your law, I said, Ye are gods? 35 If he called them gods, unto whom the word of God came, and the scripture cannot be broken; 36 Say ye of him, whom the Father hath sanctified, and sent into the world, Thou blasphemest; because I said, I am the Son of God? 37 If I do not the works of my Father, believe me not. 38 But if I do, though ye believe not me, believe the works: that ye may know, and believe, that the Father is in me, and I in him.
What did Jesus mean when he said that he and his father were "one"? He explained what he meant in:
John 17:21 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. 22 And the glory which thou gavest me I have given them; that they may be one, even as we are one: 23 I in them, and thou in me, that they may be made perfect in one; and that the world may know that thou hast sent me, and hast loved them, as thou hast loved me.
Look at this verse. Jesus wants his apostles to be one, just like he is one with God. How were the apostles and his believers "one"? Their common focus of serving the commands of God made them one. Many places in the bible, believers are referred as being of "one body" composed of many separate parts. Though they are many different people, to Christ they are one body accomplishing God's will.
explanation of "oneness" is also given in John 14:20
John 14:20 At that day ye shall know that I am in my Father, and ye in me, and I in you.