The Omniverse is a single reality.
Omniverse contains a single fraction of existence, or the existence itself.
About the Omniverse
The Omniverse holds everything that can be accessed through progressive powersets of any particular base universe (or "reality") within itself. It is the limit of archverse recursion - this can be the first limit ordinal ω, or a much higher transfinite or post-infinite, depending on the level of recursion considered.
In a sense, it can indeed be argued to contain "all of Reality", but only in the sense that those below it observe it to be as such. The inhabitants of a universe, for instance, can posit the existence of a multiverse containing every possible state their universe could be in, and a megaverse allowing for looser laws of physics, and a gigaverse allowing for even fewer restrictions, but the limit of this application must be at the omniverse if they wish to stay within the concept of "things that are real". Thus, alternate omniverses in each others' relative Paradox Space contain things that cannot be mapped from one sense of "reality" to another.
Not many entities or civilizations can achieve or even fathom exiting the Omniverse, either being denied access to the exterior by Creator or Destroyer, or simply not being advanced enough to jump through an infinite amount of spaces. However, it has been done by many highly advanced civilizations, and even some small teams, such as Satoru's crew.
The only trait every single thing within the Omniverse share is that it exists. How it exists varies in countless ways.
Beyond the Omniverse
The Omniverse exists in the surrounding emptiness known as The Outside, a void of virtual nothingness. Whatever may lie outside of these concepts is simply referred to as Beyond, one of the many iterations contained by Transcendentem.
Two Godverses orbit the Omniverse, one belonging to Creator, and one to Destroyer.