This is the first real boss of Sekiro: Shadows Die Twice, or at least the first imposing one which is likely to cause you a few repeated deaths.
You will meet it automatically since this fight is mandatory in the progression. If the guy is relatively tough at the start of the game, here are some tips to quickly overcome.
To properly prepare for the fight, you will first need to be in possession of the bell that leads to the memory of Domaine Hirata. Once inside this souvenir, you can get your hands on oil, but also on the flame thrower that can be placed on your prosthesis.
This one is in the first memory area, in the center of the campfire where several guards are gathered. Then go back to the present and go see the sculptor to enjoy your new tool.
Once back in front of the Ogre, search the surrounding area for a Gachiin Sugar, located below the area, on one of the cliffs. Just before making eye contact with the Ogre, head left to the edge of the precipice to see the item.
Once in your possession, kill the two enemies you can spy on (just before the steps leading to the Ogre) and quickly eat the Gachiin Sugar, which will make you silent for 30 seconds. This will give you time to go and kill the enemy hidden to the left of the Ogre, who could put you at a disadvantage. Still taking advantage of the candy effect, sit silently to the right of the Ogre to make a fatal blow and remove its first health bar (you will have to find the right angle and be quick to do this).
Now that he has only one life bar left, the real fight begins. Throw oil before waiting for the right time (best when he’s on the ground right after his jump kick) to use the flame thrower on him. It will then burn slowly while being stun for a few micro-seconds, which is long enough to inflict 2 or 3 blows on it.
Repeat the process while being very careful, and avoid attempting combos of more than two shots when it is not in the vapes (or even a single shot to be more careful). You can also use the ash handles quickly to chain a few combos to finish it off.