What is the difference between "non duality" and "oneness"?
These two terms are very often used interchangeably. However, they are not at all the same.
Non duality actually means "not two". Oneness means "One"
To be "not two" means that the person has reached a level of awareness where they are no longer split into two. This could mean, "good and bad", "negative and positive", "light and dark", "divine and earthly", "right and wrong", "us and them", and many other ways we tend to split ourselves into two sides here on Earth.
Oneness means we are ALL. We are complete, whole, integrated. No sides. Not even a concept of sides.
Also, theoretically speaking, a person moving into non duality, could be said to be moving into a reality where only "goodness" exists. By the same token, another person could be moving into a reality that only "evil" exists. These realities would be non dual, however, they would not be in Oneness.
One of the most difficult concepts for a person to accept when moving into Oneness is that there is no judgment of actions or persons. In Oneness, Jesus and Hitler are equal and integrated. There are no "chosen people" in Oneness. No one is chosen to be lifted from Earth and taken to a heavenly paradise. Earth Ascension does not mean a move to a reality of paradise on Earth, or Angels coming down to gather the "good people" and save them from disaster.
Oneness is complete, fully integrated.