Prime Material Plane
The 'Prime Material Plane' is the plane of existence in which Oberon resides. It is, in effect, a 'Universe' although as it is accepted that the Prime Material Plane does NOT contain "everything that is known" the technical definition of a 'Universe' does not actually apply.
The Prime Material Plane is, for all practical purposes, 'infinite' in size. It is also just one of an infinite number of Material Planes all of which inhabit the same space but are separate from each other.
The Prime Material Plane is just one of the 'Inner Planes' of existence. Adjacent to the Prime Material Plane are the Shadowfell and the Feywild (separated from the Prime Material Plane by the Shadow Veil and the Green Veil respectively. Surrounding this are the four elemental planes (Air, Earth, Fire and Water) and the demiplanes (Magma, Ooze, Ash, Ice, Dust, Steam).