While I love one night stands, from a story point of view, they don't bring much to the table. For now, "stable" bulls give me more freedom, to develop story elements and kinks over time, without having to re-add a character each time.
I think one night stands can be very hot (as fucking strangers can be seem as more slutty), and if the girl keeps going to the same guy, there's a risk of him looking like a secondary boyfriend.
I've always prefered it that way, no strings attached. Some works do feel like the girl is starting to have more feelings for the bull than the MC...
But I understand that it's easier to go to someone you already have some intimacy (I don't think there many normal ways to find someone to fuck monday afternoon or something like that. Hahaha).
Nonetheless, I hope there is some random guys throw in between the bulls sessions for some kind of variety, and also hopes Oliver starts having more personality on the sex itself, not just the talking.