Oho seems like someone’s unhappy with the decision she made. Tho it might be because she wants to be a better dancer and knew kiki couldn’t get her there. Or cause she knew Kiki wants to be the follower and somehow figured it’d work out if she left her. Or a third vague crush reason but hopefully not.
Pretty sure she's pissed that her best friend of how many years still won't confide in her.
Necessary throat clearing: not trying to continue the "does Kasumi love Sakurako" or "are they in a real relationship" arguments. (Yes, duh!)
That said, trying to come at this through the lens of authorial intent: how much of the ambiguity in the K+S relationship is the story Yukiko (still) wants to be telling, and how much of it is her being locked in by audience expectations at this point?
More particularly, I'm wondering if there are examples of manga - ideally yuri - that's successfully transitioned from multiple volumes of will they/won't they to multiple follow-up volumes of "yep, we're in a serious relationship, and all that follows." I'm wracking my brain, but the vast majority of yuri I can think of ends almost immediately once the relationship is consummated, whether that's a mutual confession, kiss, or sex. (Morinaga's My Cute Little Kitten is damn near the only exception I can think of, and that featured full-on sex within the first five chapters or so...)