The experience of oneness is a side-effect of the falling away of believed mental narrative; it’s only the labeling, dividing mind and its stories about “self” and “other” and “world” that creates the impression that things are fundamentally divisible. As you mentioned, science tells us that the separate things we think we perceive aren’t really separate things at all in any realm apart from mental narrative. It’s like becoming lucid during a dream and suddenly realizing that everything you’re looking at is really just one dream, which itself is inseparable from the dreamer.