While resolving Osseous Sticktwister's last ability, the next opponent in turn order chooses a card to be discarded without revealing it, chooses a nonland permanent to be sacrificed, or chooses to do neither. Then each other opponent in turn order does the same. Then all of the actions occur simultaneously. Opponents will know what choices opponents earlier in the turn order made, although cards chosen to be discarded this way won't be revealed until they're discarded at the end of the process.