Appearing on the Alldimensions wiki, the omnisphere was described as being "absolutely everything imaginable, unimaginable, possible, impossible, existent, non-existent, known, unknown" etc. The Omega Omnisphere is every alternate reality, every world, every multiverse, every omniverse, every Godverse, every force every being, every endless chain, every endless hierarchy, every creation, every concept, every thought, every imagined thing, ...in other words, just everything. Even other and all other concepts that involves being imaginable, unimaginable, etc..
Despite what the page claims, it would nonetheless still be within The Box, as if it were to somehow "contain the box" it would itself be contained by another version of the box. This could still however be up to debate.
The Omega Omnisphere can also be described as a sort of canvas. An infinite blank space where at some point the Omniverse had been created, a mere spot or microbe within the larger blank void. This idea stems from the 'Overmonitor' from DC comics.
If the omniverse is truly the only 'thing' within this blank space, beyond may be something neither blank nor shown as 'something', or perhaps not there at all, making us the sole totality on this canvas. However why this would be called the 'Omnisphere' is unknown.