In the Dream World, Aubrey is shown to have feelings for Omori. Because of Omori's indifference and silence, Aubrey's attempts at closeness generally go unreciprocated; only the fully developed "Look at Omori" follow-up ability is an exception. On the other hand, it is implied that Sunny has, or once had, a crush on Aubrey.
In the game files, there is an NPC with a crown of flowers on its head who resembles Sunny in Headspace. The developers most likely used this NPC for various testing purposes during the game's production.
According to Omocat, the Humphrey area is their least favorite part of the game. In fact, in the console versions, an option was initially planned that would let players blow up Humphrey with dynamite and skip the entire area. However, this idea was shelved by the co-developers.
In the 2017 trailer, interacting with the door in White Space offered the option to look through the keyhole. Doing so showed a brief moment of Kel looking at Omori with a smile. This feature was removed from the full release for unknown reasons.
According to Omocat, in the game's early stages of development, Mari was planned to be an ice cream princess. This idea emerged at a time when the Orange Oasis was meant to be mandatory for story progression.
In the game files, there is an unused sprite set for a monochrome Basil. These sprites were used for testing in one of the debug rooms called "Dev Room C." When spoken to, the Basil NPCs respond only with "Waka."
During the game's early development, it was planned that the player could change Sunny's gender to male or female. This feature was later canceled; it would probably have increased the workload, as it would require additional sprites for Sunny's real-world animations.
Early in production, extra Something battles themed around fear of public speaking and fear of attractive people were planned for the Omori Route. However, these were never added to the final game.
By interacting with the statues in Sweetheart's castle, it is possible to see different poses of the duchess. A total of six poses are displayed, and Sweetheart's iconic laugh is heard with each pose change.
Kel's name was initially going to be different during production; his name appearing as Kelsey in old development screenshots proves this.