It was no strawman....I am saying straight out that the only evidence that counts is your own actual realization....there will never be in all eternity objective evidence, for what type of evidence is there except the cosmos itself as it is... However let us for the moment imagine there is some form of non-subjective evidence available that would persuade you there is truth in the claim of union with cosmos...please explain what form of evidence it would likely be...or give an example?
I still am not sure I understand what you mean by "oneness", but, if you mean that we are part of of the universe we experience. There is a plethora of available evidence for this. Basically, the question is whether the universe exists apart from our mind experiencing it.
One piece of evidence is the experience of others explicitly described as matching what you experience. There is Hegel's idea that we make the realization that we are real when we witness others experiencing us. In other words, when another consciousness experiences us, we realize that we are a part of the universe they experience and vice versa.
Purely 1st person, subjective experience, not verified by the experience of others, is unreliable. While we might feel that we experienced a revelation, sign, or moment of higher consciousness, it is unverified and could just be merely an illusion. Now, that is not to say that subjective experience on a given occasion isn't accurate. I am just pointing out that it there is no way of knowing or supporting its accuracy when it is merely one person witnessing something.