Depending on how many OST characters there are in your story, you could easily explore a scene (4 events) with each OST character and your MC. This would create additional relationships. This is probably more likely if your OST characters have different paths in the OST (with the same theme).
The novel that I am currently listening to does this.
In this case, the MC is in a school where people train their magic talents. If they pass the school, they are allowed to become climbers which allows them to climb magic towers in search of more power (each character has their own motivations for wanting this). This is the OS concern that all of them have in common. More power.
Many of the characters in question work together on a team in the school (some are out of the school, but they all seek more power). There are easily 8 OS characters (I believe).
The MC often develops his RS stories with the other characters in question as he has a repulsion or aversion to physical and emotional intimacy. This is achieved through the OST, MCT, and RST stories.
An example of this is when another student asks him for help in a practical exam (for a class that he is not in) when her normal partner drops out. She asks the MC, because she is comfortable with him because of their other OST interactions.
Each character has the potential to have additional scenes that haven't been explored, but it just requires one of them to say, "Hey, MC, I need your help." I think this gives the MC a chance to flesh out RS stories, and it likely will fit into the OS.
Hope that helps.