When it’s time to solve the mystery of how Oikura’s mom disappeared from a locked room, it’s not surprise that Ougi shows up to cramp Hanekawa’s style. For someone whose face is essentially a mask, she sure doesn’t mask her contempt for Hanekawa and her large boobs, which she feels are exclusively responsible for stealing Araragi away from her.
As usual, I’m not sure how much of what Ougi says is serious and comes from her heart, because I’m still not sure she has a heart, and isn’t some kind of strange construct or apparition, in contrast to all the flesh-and-blood girls in Araragi’s life. She says all the things a jealous underclassmen who likes him would say…but does she really mean them?
I hope we’ll find out later. In the meantime, we have an arc to conclude! And conclude it does, with Hanekawa answering Ougi’s challenge and coming to the same conclusion as to what happened to Oikura’s mom. That leaves Araragi as the only one yet to realize the truth…and it’s a truth Hanekawa would rather Oikura never be told and never know for the rest of her days, not matter what immediate benefit could arise from telling her.
Still, she agrees with Ougi that it’s something Araragi must figure out for himself and make his own choice. They start offering subtle hints, and he keeps coming to the wrong conclusions, so they give him less subtle hints (over forty of them!) until he’s finally got it: Oikura’s mother starved herself to death, and for two years, Oikura took care of a corpse, until it eventually decomposed into nothing recognizable, giving the impression she disappeared, while she actually “evaporated”, like boiling water.
It is indeed an awful truth, and one Araragi and the other have no idea how Oikura will react to. But Araragi decides he’s going to tell her. He’s through looking past/overlooking Oikura, as he has for the last six years, as she overlooked her dead mother for two. He’s going to see her, look her straight in the eye, and tell her the truth. It’s a long walk back to his apartment, and the sequence of camera shots in the intensifying sunset make that walk a beautiful occasion.
Oikura takes the news far better than Araragi expected. More importantly, learning the truth (or perhaps, having it confirmed by someone else) made it that more real, and that much more releasing. Turns out Oikura is moving to a smaller municipal condo, and transferring out of Naoetsu High. But she went back to her class anyway when she knew Tetsujo was on leave, hoping something might change. In the end, Oikura is smiling, but not demonically, before the bright sunset. And the brightness isn’t hurting her.
Now that things avoided have been remembered, things at a standstill can move again. Because what was done with the truth was more important than discovering it, Ougi later concedes this particular case was her loss, also admitting she was wrong that Araragi would turn tail and run like he had in the past. But helping Oikura find change helped him to change too.
Oikura visited Senjougahara and they made up, and she left to start her new life. But not before taping an envelope under Araragi’s desk. This time, it had something in it: several pages. What exactly it was is kept a mystery (which I like), but whatever it is gives Araragi a laugh, so I like to think it’s a reversal of the message the earlier empty envelope sent.