This is more in the realm of philosophy and spirituality than of science.
Buddhism counts Nirvana as the ultimate spiritual goal, the cessation (or "blowing out") of temptation (or passion), aversion and suffering. The state of "oneness", or more accurately "mindfulness", is an individual journey, marked by intangible mileposts, science is neutral on such concepts. With respect to those who observe such inward spiritual philosophies, it would be difficult to argue that such a state does not exist. The Buddha himself is said to have reached the pinnacle of "Enlightenment". It is from the teachings of the Buddha that the path(s) to the states of Nirvana are set forth.
That said, there may be much to be found in the study of brain activity of those achieving such states of "being".
There is much more to this than can be fully articulated here, but perhaps this brief overview addresses your question.