Chained Ogre is the first boss in the Sekiro game who is located in the Ashina outskirts.
You can follow the steps below to make him dead.
How to defeat Chained Ogre:
Kill three minions and jump over the wooden wall.
Silently sneak near the boss and back stab him. Then use the flame vent and slash him with the help of a sword.
Wait for the fire to go down and then ignite him again with a flame vent. Repeat the same combination of igniting and slashing a few times. And Jump on his head for the final blow.