Oneness, n.
The state of being one; singleness in number; individuality; unity. Our God is one, or rather very oneness. Hooker.
Shades of meaning
Fernald has no entry of its own for oneness; he lists it under union.
Where he writes about it:
Unity is oneness, the state of being one, especially of that which never has been divided or of that which can not be conceived of as resolved into parts; as, the unity of God or the unity of the human soul. — from union