They're def both at fault, but its good that they both decided to end it, because it straight up wasn't working for either of them. Something interesting to note was that while Nozaki admitted that he was at fault that he himself didn't do enough, he also noted that he only was able to see Oshiko as a friend. Therefore, I think that him not doing much in their relationship was truly communicative of what he felt about them dating: just not feeling enough to do something for someone he allegedly likes. With Miharu iirc (its been a while since I first started reading), he chose to move to benefit her and whatnot, hence communicating that he does value her highly. Finally, I think that this whole situation was neccessary to an extent, especially for someone like Nozaki. He has a lot of flaws, obviously, but he needs proper motivation to change. Nozaki's core issue is that he is passive and lets others dictate his pace, when he is his own person and should learn to stand for himself and be able to live with his own actions. If he went straight into a relationship with Miharu instead, I feel like he would've had the same issues.