That's kind of the implication here, her roster actually torments her, and in trying to save the students, she's both cursed to not talk about their deaths, or even the fact that she knows about it, and know she might be able to change their time of death.
She's changed some, but its made those students she saved resent her. The class president used to be close to her, and now she hates Oka to the core. Last episode made it seem tragic, but this episode revealed that Oka was trying to be well-liked by her students to the point that she didn't even know if she was happy herself.
Hugo is going to die in the elven forest attempting to invade it, basically. Katia's wasn't meant to be when she did die in the series, but was set for five years from now. We haven't seen if it has changed. Oka likely saved a number of them from premature deaths as children. So it isn't set in stone, and I would think of it more like the "current most likely outcome," rather than a defined fate. I wonder if Fei's used to say, "egg cracked open and eaten by a spider."
My guess is that D doesn't like Oka very much, and definitely did this to her on purpose. She took her desire and twisted it all around like an evil djinn. D is a sadist, and maybe she even suspected that Oka was faking quite a bit of her true personality. The way Oka ended up using the roster produced the opposite outcome that Oka was hoping to accomplish. For all we know, D is lying to her through the roster to manipulate her and make her suffer. Or maybe not, because truth is always so much more painful than lies.