I've written recently about 'oneness'- distinguishing it from what I believe to be the correct understanding of Christianity.
But I should clarify that the teaching of oneness as an ideal always entails duality in practice: monism is always really dualism.
And this has been the case since the very earliest, pre-Socratic philosophers and Plato and the Neo-Platonists; Hinduism and Buddhism, and the Platonic-influenced but mainstream versions of Christianity... Some of these claim variously to be monisms, but all actually are dualisms.
This can be seen in the attitude to change and changelessness. All assert that ultimate reality is changeless, outside of time, unified and perfect.
But all are forced to account for the fact that the world as we know it is changeable - characterised by with disease, decay and death - and im-perfect.
This is regarded as an illusion (maya), a temporary misunderstanding and/or misinterpretation (eg. due to sin, perhaps due to a 'fall') - nonetheless, this claim only kicks the can; because if all is truly one-ness and perfect - where/ why/ how, then, does illusion come-from?
Sooner or later, some kind of dualism of reality must be introduced; and always is introduced.
Total reality is - in effect - divided into two abstract categories; one true real-reality and the other erroneous mere-appearance. The key assumed fact - needed to complete the basic picture - concerns the source of that error.
The only alternative to dualism (or the ultimate two-ness of reality) is not oneness (which simply does-not-work), but pluralism: more-than-twoness. I am a pluralist, and my ultimate category is Beings, which are living/ conscious, eternal, many, and remain them-selves - through whatever transformations they undergo. Other pluralisms are possible (e.g. pluralism seems to be the spontaneous assumption of children and hunter-gatherers; and is proposed theoretically by William James and Mormon theology), but the assumption has never been popular among philosophers; and is regarded as a mere mistake in reasoning by the vast majority. Regardless, I believe it to be true!