|WARNING: This article contains spoilers for the main story of OMORI.|
Omori starts off inside White Space. There are several things in the main area of White Space, namely: A lightbulb, a laptop, a sketchbook, Mari's cat named "Mewo" and a door to Headspace.
Red Hands roam outside of White Space's main area, and bumping into them results in Omori being teleported back to the main area.
After the death of Mari four years ago, Sunny's repression of the truth began with the creation of White Space, as well as his alter ego Omori. The one constant of Head Space, White Space serves as a safe-if-sad place for Sunny to escape from harsh reality and responsibility. The only thing that ever changes is the lightbulb, a curious object filled with pitch-black darkness instead of light. If a lit lightbulb represents "the birth of an idea", then a black lightbulb is "the repression of an idea" -- it serves as a visual metaphor of Sunny's repression of the truth, and its darkness pulses stronger the more the truth begins to leak out in the Headspace, often in the form of Something.
White Space is where Sunny takes on the form of Omori to explore Headspace, and for much of the game, this and its nature as a safe place are the only purposes White Space serve for the plot. But as the game nears its end, White Space gains more significance.
In most cases upon Sunny entering White Space, he immediately fuses with Omori in a cutscene -- but after Sunny has reconnected with all of his friends in Faraway and found the strength to face his fears, the player is suddenly controlling Sunny within White Space while a separate Omori watches and shows no apparent desire to fuse with Sunny this time. With no door to Headspace present in this case, the only option is to inspect the Lightbulb. Feeling ready to confront the truth, Sunny smashes the lightbulb on the floor and ends his repression, to the shock of Omori -- Sunny is then expelled from White Space into Black Space and thrown straight into his final confrontation with Something.
Following this and his ill-fated meeting with in reality, Sunny finds himself within White Space again. What few objects were within White Space before are gone, leaving only the border of the room, the cord that used to hold the lightbulb, and Omori himself who stares at were it used to be. As Omori turns to face Sunny, the border and cord vanish, leaving only Sunny and Omori in an empty white void as the two begin to fight for control over Sunny's mind.
Should Sunny completely ignore the real world outside of his house on all three days, he will have no desire to find the truth, and thus Omori will have no reason to be hostile. Sunny is accosted by Something as he sleeps, but Omori saves him and they fuse -- this time permanently as Omori takes full control of Sunny's mind.
White Space in this case has a couple of subtle changes. The laptop will now serve as a portal to an altered Black Space, and the lightbulb will continue to pulse erratically with darkness -- but if Omori and his friends defeat the forms of Something in the Lost library, the lightbulb's pulsing will cease. Notably, Mewo will be missing entirely if the player chose to cut her open in the Punishment Area.
White Space was the first thing Sunny's mind created after Mari's death, it is described as "an emptiness, a home without warmth. A place to survive, but not to live." from the Branch Coral found in Deeper Well.
Daddy Longlegs explains in the Lost Forest that Sunny's desire to explore his mind became strong enough to create a world for himself, so he created Headspace, an idealized world of his own.
There he found short lived peace and escapism from the truth. He started shutting himself in start to live within his own world where nothing went wrong. Even though he got accustomed to Headspace after a while, his mind still found ways to confront him.
To deal with this he created Omori, an alter ego. At night, he assumes his identity and leaves Sunny behind.
- White Space's main area is laid out exactly the same as Mari's picnics.
- At the beginning of the Two Days Left dream, Omori will instead wake up in Neighbor's Room. Attempting to reenter White Space will result in him seeing Something through the door hole, before noting that the door is locked.
- If Omori stays in one place for a certain amount of time, he will sit down.