Oneness (concept) refers to the experience of oneness and monism. The concept appears in multiple religious and mystical belief systems/philosophy, most notably in Hinduism.
Oneness in Christian theology is an attribute of the Christian deity put forward in contradistinction to trinitarianism. Oneness of God means that Father, Son and Holy Spirit, while all fully divine (in contrast with the lesser status of Son and Spirit associated with some beliefs), are not separate persons but expressions of the one God (alpha omega =being the first and last). Oneness Pentecostalism is a specific sect of Pentecostalism proclaiming the oneness of God. There are others (not Pentecostals) who believe in the oneness of God: see also Unitarianism, Sabellianism, and Nontrinitarianism.
Divine simplicity