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.
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.
Maybe the OC has different values from what is presented in the 'Rias DxD' fanfic. For example, if the fanfic has a lot of over - the - top or unrealistic elements that go against the OC's idea of a good story, it could lead to dislike.
It could be that the OC creator simply doesn't like the typical tropes or storylines often found in Rias fanfiction. For example, if the fanfictions tend to be very formulaic or over - use certain clichés, the OC might be a symbol of a break from that. Also, perhaps the OC has a specific backstory or purpose that is at odds with the general narrative of Rias fanfiction.
The main thing is the new relationships. When an OC is paired with Rias, it creates fresh dynamics. We can see different sides of Rias's character through her interactions with an unknown element like the OC.
There could be the other members of Rias's peerage. In some fanfictions, they might be forced to choose sides between Rias and the OC. For example, Akeno might be torn between her loyalty to Rias and her growing respect for the OC's abilities.
A great fanfic idea could be that Rias and Riser are sent on a joint mission to the human world. They have to blend in as high school students. This would be a comical and interesting premise as they struggle to adapt to human life while also dealing with their own relationship. Rias might be more empathetic towards humans, while Riser could be more arrogant at first.
One possible fanfic could be about a friendly rivalry between Rias and Riser that turns into a mutual respect. For example, they could start competing in various magical challenges within the Highschool DxD universe. This would show their growth as characters and how they overcome their initial differences.