The 'alternate' path through OMORI, the OMORI ROUTE presents a new challenge to OMORI players. Focusing more on HEADSPACE and shutting out the real world almost entirely, the OMORI ROUTE is triggered by refusing to go outside as SUNNY whenever KEL comes calling. Refuse all of KEL'S attempts to get SUNNY out of his house and you'll be on the OMORI ROUTE when you reach the end of the game.
Though you can play the OMORI ROUTE during any playthrough, playing through the SUNNY ROUTE first is probably wiser, as it gives you a more complete idea of the overarching story. The OMORI ROUTE is also more challenging, and better tackled once you have a thorough appreciation of OMORI'S combat system.
PROLOGUE
The PROLOGUE of the OMORI ROUTE is exactly the same as the PROLOGUE of the SUNNY ROUTE. If you want to save time it's not a terrible idea to create two save files once you reach the end of the PROLOGUE, then just pick up on the second file when you're ready to start the OMORI ROUTE. (Though the PROLOGUE is fun to replay.)
THREE DAYS LEFT
THREE DAYS LEFT begins as it does in the SUNNY ROUTE, with KEL pounding on the front door. If you want to experience the OMORI ROUTE you'll need to answer the door, then choose 'DO NOTHING' after checking the front door. Go back to bed and KEL will be gone by the time SUNNY wakes up.
Sitting in SUNNY'S room is a TO-DO LIST, right beside the door. You'll need to complete the chores on the list each day to progress the OMORI ROUTE. Very exciting stuff. This time you need to sweep the house, which equips SUNNY with MOM'S BROOM. Move throughout each room of the house, stand on the light brown lumps you find, and interact with them to sweep them up. (SUNNY will refuse to enter one of the rooms, down the stairs from the bedrooms. Don't worry about it.)
The game will let you know once you've done all the sweeping, and SUNNY will pass out. From this point you'll need to navigate the same horror-filled version of the house as in the SUNNY ROUTE, guiding SUNNY back to bed. Putting him to sleep will send you to WHITE SPACE. (Interestingly, you can have OMORI STAB himself in WHITE SPACE to wake SUNNY up. Walk around the house and you'll see a few more things.)
The remainder of THREE DAYS LEFT takes place in HEADSPACE. The trip to SWEETHEART'S CASTLE is unchanged, aside from the visit to the KEEPER OF THE CASTLE. If you stand still on the red tile outside the KEEPER'S room you'll get a different scene for the OMORI ROUTE.
TWO DAYS LEFT
TWO DAYS LEFT will begin just as it did in the SUNNY ROUTE, though if you turned KEL down on THREE DAYS LEFT he won't be knocking on your door when SUNNY wakes up. Head into the bathroom, have SUNNY brush his teeth, and check the TO-DO LIST in SUNNY'S room. This time SUNNY needs to do the dishes. Just keep choosing items from the list of dirty dishes until you run out. Exciting.
SUNNY decides to have a bath after the dishes are done. Head up to the bathroom and check the bathtub to give SUNNY a scrub. Another creepy sequence will follow, one also found in the SUNNY ROUTE, and SUNNY will need to face off against SOMETHING to free himself. Once the battle is over you'll have control of SUNNY again. With the house dark you can explore and find a few spooky moments - mirrors and the fridge jump to mind - though ultimately you just need to send SUNNY back to bed.
HEADSPACE is the same as usual, in that everyone has vacated VAST FOREST, though rather than meeting MARI to the north of the stump you'll find KEL instead. Following him will allow you to skip the misty boardwalks that are a party of the SUNNY ROUTE, taking you straight to DEEP WELL. Everything in LAST RESORT, UNDERWATER HIGHWAY, DEEPER WELL, and HUMPHREY goes pretty much the same as in the SUNNY ROUTE.
The trip to BLACK SPACE is the same until the very end. SUNNY'S confrontation with his past does not go well, and it doesn't take long before OMORI retakes control of the game. Things end peacefully, but... also not.
ONE DAY LEFT
SUNNY awakens, and though the house looks the same there's one very obvious difference... and it'll follow you from room to room. Enjoy that. Brush SUNNY'S teeth, then perform his final chore from the TO-DO LIST: Organize his old belongings. You'll have a box of assorted stuff, and three boxes - TOYS, BOOKS, and TRASH - to put them in. You can throw each item in whichever box you like. It's an easy process, though very tedious. You'll receive an achievement for completing the TO-DO LIST over the last three days.
Things get progressively spookier in the house once you're done with SUNNY'S chores, though there's nothing else to see of particular note. Hop in bed...
... and OMORI will be back in WHITE SPACE. This time one of the red hands will be positioned in front of the door, and interacting with it will teach OMORI RED HANDS. HEADSPACE awaits.