Suppose not many folks here care too much about Okai, but she is having a 2nd vocal nodule surgery (probably done it by now as she was already in hospital getting ready for it).
https://www.instagram.com/p/BiyZhecFUdT/Not going to speculate 'cos I don't know her life style nor nature of her condition, but her singing style (all chest voice) and improper techniques probably didn't help her cause in this case, if anything. She had like 2-3 UF events + maybe 4-5 TV appearances for any type of singing for the past year (after C-ute disband about a year ago).
Going all loud and full chest voice have often made Okai's vocal to have extra oomph - when compare to other H!P singers -, but you can see how someone like Suzuki who sings in mixed voice for high notes doesn't appear to have as much issue. They have more or less the same vocal range, but it comes out more easily from Suzuki and it barely ever cracks. Obviously different bodies, so it can't all be pinned on "just" techniques.
I'm more curious on why she is so quick on getting the surgery done in the first place. It's her body and it goes without saying that she can do whatever she wants. But again a plenty of singers in the past have lived with chronic vocal nodules and some never did the surgery in fear of something going wrong (ala Julie Andrews). Yeah it will make one's voice often hoarse and make it crack way more often, but it didn't stop someone like Freddie M (for like last 10+ years of his career). Okai is no Freddie, just giving an example. I'm sure there are many others, usually old-timers, but his is the most famous example I know.
She may need to stop going all chest voice on high notes all the time (eps. on notes near at the top or a bit beyond her range) and properly learn falsetto and mixed voice to ease the strain after this 2nd one.