The concept of emptiness, or Sunyata, is a Buddhist idea which basically states that nothing exists in and of itself, but exists in relation to and as a part of everything else.
The reason I group oneness and emptiness into the same category is because they fundamentally equate to the same thing.
We carry the illusion that we are separate from everything and thus we suffer, feeling ourselves to be fragmented and out-casted from the whole.
In reality we are fundamentally one part of one infinite movement. We are inseparable from the whole, in short, we belong.
Think of a dog barking. We imagine the dog to be separate to us but the dog barking occurs within our perception. It is one unified movement. The perceiver, the act of perception and the perceived are one holistic phenomenon.
Full realization and embodiment of this opens the doors very wide and very quickly as the ego, whose main agenda is to ensure its individual survival, is vanquished and the mind is submerged into oneness.
From this oneness our individuality within it can emerge, we find our place in the whole and come to know and love both ourselves and others as expressions of the light, expressions of the one being in form.
It is like our whole life up to this realization has been a frantic running away from the silent oneness of being, from ourselves, from everything.