One possible storyline could be that the Si OC is a shinobi from a village that has been wronged by Konoha in the past. They see Naruto as a symbol of Konoha's power and thus direct their hatred towards him. For example, if their village was raided by Konoha in the past for some reason, they hold a grudge against all things Konoha, including Naruto.
I'm not really sure of specific fanfics where the self - inserted original character (Si OC) hates Naruto. You could try searching on major fanfic platforms like FanFiction.net or Archive of Our Own. Look for tags related to 'original character' and 'hatred towards Naruto' to narrow down your search.
Maybe the OC has a completely different personality or set of values from what is often portrayed in Rias fanfic. It could be that the OC has had bad experiences related to the typical tropes or themes in those fanfics.
If the oc hates it but still wants to give it a chance, they could try to find a different take on the rias fanfic. Maybe look for ones that are more in line with their own values or interests. For example, if the oc is interested in action, find a rias fanfic that has more action elements rather than just the typical romance. Another thing to do is to try and write their own version of a rias - related story that they would like, incorporating the elements they enjoy and leaving out the ones they don't.
In some fanfics, the idea of Naruto hating Hinata might be a plot device to create drama. Maybe the writer wants to explore a different kind of relationship dynamic than the canon one. It could also be that the writer has a unique interpretation of the characters' personalities and how they might clash in certain situations.
Maybe it's because Aphrodite's powers and the way she uses them are at odds with Naruto's own beliefs. Naruto believes in self - improvement through training and facing challenges head - on. If Aphrodite uses her love - related powers in a way that seems unfair or goes against Naruto's sense of justice in the fanfic, he would dislike her. Like if she uses her powers to manipulate people's emotions for her own gain, which is something Naruto would oppose.
The OC can simply avoid reading any more of that fanfic. There are plenty of other stories out there in different fandoms or even within the 'High School DxD' fandom that might be more to their taste.
Well, it could be that Naruto has been betrayed too many times. Maybe he discovers that some of his closest friends have been using him all along for their own gains. So he starts to hate everyone, and the story could be about his journey of isolation and self - discovery as he tries to come to terms with his new feelings.
Often in these fanfics, Sakura might make fun of Naruto's dreams or goals in a very mean - spirited way. This constant put - down could lead Naruto to hate her. Also, if Sakura were to side with Naruto's enemies in the fanfic for some reason, like being brainwashed or having a hidden agenda, it would be a likely cause for Naruto's hatred.
One possibility for such a fanfic is that Naruto has been through some kind of trauma that makes him view Sasuke and Sakura differently. For example, if Naruto was betrayed by someone he trusted deeply who had similarities to Sasuke and Sakura in some aspects. It could change his perception of them completely and fuel his hatred in the fanfic.