Oneness does not mean singleness, but rather it implies two or more. The Lord said that “I and my Father are one” (John 10:30). Hence at least two, plural and not singleness of person’s or in this case a single God, but Gods plural who share a oneness, one purpose.
Know ye therefore that they which are of faith, the same are the children of Abraham. And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, in thee shall all nations be blessed. So then they which be of faith are blessed with faithful Abraham. (KJV Gal 3:7-9)