Rulings

If the target creature is an illegal target by the time Ob Nixilis's last ability tries to resolve, the ability doesn't resolve. No player draws two cards. If the target is legal but not destroyed (most likely because it has indestructible), its controller does draw two cards.
If Ob Nixilis leaves the battlefield before his last ability resolves, most likely because he only had 2 loyalty when you activated the ability, his first ability won't exist to trigger when the creature's controller draws two cards.