There are many levels of “oneness”, but in short, the initial “ego death”, or self-realization, is a jump from experiencing sensations to becoming sensations. In the separated state, there’s an illusion that there is an entity (ego, or “you”) that sees, touches, smells, hears. There’s a division between the subject of perception and the object of perception. When this entity is no longer there, there is just perception, in itself, without the object/subject division. So, there’s seeing, but no “seer”, hearing but no “hearer”, etc.

The subject of perception is not real, it is thought-created. When the jump happens, thought stops occupying the central stage - there’s thought, but no “thinker”. So all of these perceptions taken together - seeing, hearing, sensing, thought - are now “you”, because the old “you”, the one who experieneced all that, is no longer there. So people often describe this experience as “There is no me - I am everything”. In this experience, there is nothing you can point your finger to and say “this is me” or “this is not me”. When the subject of perception disappears, all objects of perception become you, there is no object/subject division anymore. This is an ongoing experience.

Similarly, you can no longer say that “you have a body” or “you have a mind” because the subject that “had” those things disappeared. You can say “I am the body” or “I am the mind”, “everything is mind” etc. but not “I have a mind”.