This Oneness is what the followers of Advaita(Non-dualism) call as Brahman or the Buddhists call as Nirvana(Nothingness). I have seen arguments that Plato was also influenced by Eastern philosophy, but I don't know much about it.

I was interested in Advaita for a long time, and used to read Upanishads which expound about it. but recently I have come across a different school of thought, which calls this Oneness/Brahman/Nirvana as Self/I. Ramana Maharishi who died 70 years back was a recent proponent of it. but I have seen references to it from different cultures too, like the hebrew word YHWH meaning Self, and the Persian word for God "Khuda" meaning Self in Old Persian.

I recently came across Bernadette Roberts, who is a Catholic Nun, who seems to have not heard of the concept of Self from Advaita/Hinduism, but have reached to similar opinions about Self from her own contemplation/Prayers in her monastery. and in this video she explains it from her perspective as a Christian Mystic. She also has some books, which I haven't read. https://www.youtube.com/watch?v=Nb5496uQ3lk