My memory is a little fuzzy so could someone remind me. Didn't she buy a house so she could live with her last boyfriend without asking him about it beforehand? I think this is why the last one dumped her. She also seems to have a real problem with clinginess, not physically but wanting to be around him all the time.
Looking back, Oshiko gets introduced in chapters 74-91. In chapter 78 she mentions buying the apartment and getting dumped but not the circumstances that led to either, unless that comes later in the story.
Even back in these early chapters, she's pretty flirty and direct with Nozaki, quickly calling him by his first name (I forgot about that) so I imagine her clingyness was probably a big reason she ended up single again.
She mentions her hobby is hospitality, so unless that was part of her backstory that the author rejected, her clingy nature at least seems to come from good intentions. Maybe she just has issues picking up on those social cues and boundaries, which isn't too dissimilar from Nozaki, just in a different way.
It would be interesting to see Oshiko's inner thoughts during all this.