OMORI has two main endings, as well as a neutral ending with three different variants.
Every main ending is affected directly by the game's final choice after Sunny's defeat in the Omori battle.
If Sunny chooses to keep going after being struck down by Omori, he will gain the resolve to play his Duet with Mari for the last time. Sunny then absorbs Omori and wakes up in reality, where he seeks his friends and confesses the truth behind Mari's death.
If Sunny gives up against Omori, Omori will absorb him and hijack his body, before returning to White Space. He prepares to go out on another adventure with his friends, while in reality, Sunny commits suicide by jumping off of the hospital balcony.
Secret Flower Ending
In this cutscene, Sunny and Basil, alone in their hospital room, smile at each other as their Somethings fade away.
If Sunny chooses to run back home instead of helping Basil, he will get these endings. Additionally, if the player is on the Hikikomori Route, they will be locked to these endings.
If Sunny abandons Basil after learning the truth, he can pick up another steak knife in his kitchen. If he does so, after he visits White Space for the last time, Sunny will never wake up; he will have stabbed himself in the chest in his sleep and bled to death. The ringing of a phone can be heard over the credits.
At any point after completing Black Space, Sunny can choose to kill himself by opening the menu and selecting the Stab option. While he will noticeably hesitate, he will eventually stab himself in the chest and slowly bleed to death on the ground. The ringing of a phone can be heard over the credits. This can also be done on Moving Day.
If Sunny abandons Basil after learning the truth, but does not pick up the steak knife in the kitchen, he will wake up on Moving Day. He will head outside as various Movers help move the last of their belongings out of the house, and will get into his mother's car, still stalked by Something, implying that he will never get over his trauma. Sirens will additionally be heard over the credits, suggesting that, since Sunny never intervened, Basil succeeded in killing himself.