William James (1985 - 1910), founder of American psychology, in his classic text, The Varieties of Religious Experience, described Oneness and its relationship to mystical experiences in the following way:

“This overcoming of all the usual barriers between the individual and the Absolute is the great mystic achievement. In mystic states, we both become one with the Absolute, and we become aware of our oneness. This is the everlasting and triumphant mystical tradition, hardly altered by differences of clime or creed. In Hinduism, in Neoplatonism, in Sufism, in Christian mysticism, we find the same recurring note.”

The idea of Oneness, that the self is inextricably intertwined with the rest of the universe, can be found in many of the world's philosophical and religious traditions. Oneness provides ways to inwardly sense the self as fundamentally connected with other people, creatures, things, and spiritual realities. This is a challenge to Western hyper-individualism and its tendency toward self-centered behavior.

May they all be one, as you, Abba God, are in me, and I am in you, may they also be in us.
John 17: 21

Jesus said, "When you make the two one, you will become like the sons of man, and when you say, 'Mountain, move away,' it will move away."
 Gospel of Thomas, Saying 106 

The eye through which I see God is the same eye through which God sees me;
my eye and God's eye are one eye, one seeing, one knowing, one love.
Meister Eckhart