Can Zoro Use Observation Haki. Hes not fast enough. Perhaps the most important thing that Zoro learned during the two-year timeskip is Armament Haki.
This form of Haki allows a person to use their spirit as an invisible armor and increase both their offensive and defensive capabilities. Oda probably didnt have haki planned out at the time. Nov 29 2021 Chapter 1033 confirms that Zoro has unlocked ADVANCED Conquerors Haki.
Another proof that Zorro couldnt use Haki at that time is that he was able to cut through Daz Bones metal which is actually possible with enough skill but he couldnt cut through Enels lighting.
But not the haki that we know today. He was first noted to have used it by Kaido after the Emperor was struck by Zoros Kyutoryu attack. Zoro is certainly physically stronger and a better swordsman but Law would beat him in a fight because of the nature of his abilities. Zoro is a swordsman and he does not have any devils fruit power he always has to rely on his strength as a swordsman he needs to protect his sword using Haki.