According to most definitions, the mystical vision and experience is about the mystic attaining first a vision of the oneness of everything - that all divisions and distinctions, including time, are illusory; and then attaining a personal unity with that oneness.
The idea that time is illusory, and that reality is not-time, outside of time, time-less; has no past nor future but Just Is... This idea is, I believe, the crux of the mystical experience.
And the belief that one has experienced not-time, an 'eternal present', is the root of the mystical interpretation of oneness. It is the basis for the inferences drawn from it.
My interpretation is that mystics have experienced... something in relation to time; but they have never experienced not-time; because time Just Is, it is a basic metaphysical reality.
At a more 'common sense' level - I find it rather convincing to argue that not-time could not be experienced, because there can be no experience without time, and no memory of that which is not in time without duration.
But if the mystic did not really experience not-time - what then did he experience? What was the experience that the mystic interpreted as not-time?
My understanding is that the truth of 'not-time' is a very short period of time, averaged.
When a sufficiently short period of time is experienced, then essentially 'nothing' happens during this time - and this is particularly the case when what-does-happen is averaged into a single 'value'. Then the person feels as if time has ceased; and from this (apparent) stillness he can view every-thing that he can view in a still panorama - all as one picture of 'all' reality.
Of course he does not actually perceive everything! But the point is that all which he does perceive from his small-and-averaged time-slice is a part of the one still image.
From this experience, the oneness mystic has an apparent confirmation of the oneness of all things and all times.
But in fact, the experience itself is merely an artefact of the process of deciding that the shortest possible time-slice, from-which all change has been averaged-out - is 'true' knowledge of reality.
Analogy: A photograph is not an instant outside of time, it is a slice of time: an average picture of what happened in in the field of vision during a particular time slice (defined by the shutter speed). The real world was moving, but not much over the time-slice.
Because it was a relatively-small time slice (compared with normal human experience), and provided nothing depicted was moving too fast; the averaged picture on the image is not very blurred, and may appear as if time had been frozen.
And even when something is moving fast enough to blur (as in almost photographs of a waterfall) what results is a still and sharp picture of a blur.
A camera has a partial and perspectival view of reality - as does the human mind - neither can perceive everything in existence at once (nothing like!). But if it is assumed that everything-being-seen is every-thing - then the field of view is everything (by definition).
Then the deletion of time (by averaging of the time slice) apparently makes every-thing-one.
In conclusion; my understanding of the common mystical experiences of oneness, are that they are attained by those who believe in the truth - or at least the coherent possibility* - of oneness - and who are 'looking-for' oneness; and that they are actually a memory of a small slice of time which is being interpreted as a time-less instance; and that the attribution of oneness to every-thing in existence is an artefact of the 'field of vision'.
*Oneness is Not a coherent belief, because the fact that it is a belief entails that there is Not oneness: we can only 'believe' from a prior position of separateness - which ought not to be possible if everything is one. The common notions of 'ultimate' oneness being concealed by 'illusion'/ maya does not make sense either - because if all is one, illusion should not be possible in the first place. And, anyway, how could we know of oneness if illusion prevailed? It seems to me that although the belief-in ultimate oneness has been common in humanity (e.g. modern Hindus, Buddhists and many New Agers); like many common beliefs it does not cohere, even superficially... unless further assumptions are being smuggled-in unacknowledged, which is, in fact, exactly what happens.
NOTE: The above is an example of that type of philosophy called metaphysics. My assumption is that fundamental assumptions concerning the nature of reality are what shape evidence - not the other way around. Thus the 'evidence' of 'oneness' in mystical experience is understood in terms of prior assumptions. If the prior assumption is of oneness, then this type of experience is regarded as evidence of oneness. But if one is a pluralist, like myself - who believes that time is a fundamental; the mystical experience of oneness is interpreted in light of those qualitatively-different assumptions.
This 'un-disprovability-by-evidence might be assumed to imply that metaphysics is arbitrary, and one set of assumptions is just as good as another; but this is not the case - because different metaphsyical assumptions can be examined in terms of their coherence. Thus I have tried to argue above that the experience of oneness is - in the first place - not a proof of oneness; and in the second place that oneness is an incoherent belief. The incoherence of oneness can be, and is, explained on the basis of further assumptions being introduced - but all of these contradict the prima assumption of oneness - therefore I argue that oneness is incoherent and disproves-itself.
The same also applies to mainstream/ orthodox/ Athanasian Trinitarian Christianity; which tries to argues simultaneous oneness and plurality. Insofar as the Trinity is taken to be one and every-thing, it is also incoherent and self-refuting; and attempts to cover this by asserting simultaneous plurality are also covert introduction of further unacknowledged assumptions.