In Theosophical cosmology the Oversoul corresponds to the Monad — the divine spark that descends into matter at the beginning of the soul's evolutionary journey and returns, vastly enriched, at its completion. The Monad is the seed of divinity implanted in each individualised soul — a spark of the universal fire that retains its individual character across the entire arc of evolution from mineral to divine. Where the soul develops across lifetimes, the Monad develops across evolutionary cycles — across the entire span from unconscious matter to fully conscious divinity.
In the Seth Material, the equivalent concept is the entity — the larger being of which any individual personality is one "fragment." Seth describes the entity as a vast consciousness that simultaneously expresses itself through multiple personalities in different times and dimensions, each living out a different "probability" of experience. The individual you — the personality reading this — is one of the entity's current experiments in consciousness. You are not the only you that your entity is running.
The concept of the Oversoul invites a radical reframe of individual identity: the "I" that you take yourself to be is real but not ultimate. It is the entity's way of experiencing the physical world from a specific, limited perspective — a perspective that is valuable precisely because of its limitation, its specificity, its here-and-now quality that the broader entity cannot have from its own larger vantage point. You are the entity's eyes and hands in this particular corner of physical reality, at this particular moment in time. The entity chose to be here, through you.