So Ooe is very, very heavily based on and then there were none, to the point that the main character reads And Then There Were None at the start of the VN and then multiple times throughout the VN directly compares his situation to the novel.
I was not expecting, in the ending of the first route, to directly go through the cast of And Then There Were None and Ooe and say "this is the character from And Then There Were None this character from Ooe was based on". You spoil yourself on Ooe by having read And Then There Were None.