OX in ZX has a magical purple Z-Saber.
Omega was the only boss resembling difficulty, there.
They ratched up his difficulty there.
Zero 3: He sometimes stops moving for a second, only uses his seven slash combo when low on health and even then, rarely, and he doesn't self heal. He's pretty easy.
ZX: He's absolutely ferocious. He spams that seven slash combo like no tomorrow, he can self heal AND he's got little to no delay between attacks and movements.