Ame-no-Habakiri beheading Orochi is something we can be very confident about.
In mythology, Habakiri kills Orochi. Habakiri means "snake slayer", etc...
Orochi has had 7 out of his 8 heads chopped up and now has 1 remaining.
Given how easily defeated scabbards beheaded Orochi.
It would be very bizarre for Oda to try and have Zoro fight Orochi and NOT behead him given how easily injured scabbards did it.
As to who will fight and kill Orochi. It will be whoever uses Habakiri in combat.
My best guess is Momo.
Momo is going to have a moment thanks to his Dragon fruit and an additional moment thanks to Habakiri.
Personally, I believe that his dragon moment will be done by accident. However, thanks to that moment, Momo will finally realise that he **isn't** weak and that perhaps he **is** ready to use Habakiri.