Stage I
Oceiros starts the fight with a couple of basic attacks. His staff bash comes out quickly, so dodge early if he pulls his staff arm back. The tail whip takes longer, so that’s an attack you need to be patient with. Hold your dodge until the last moment to avoid the damage and still have time to pummel the boss.
Put some distance between yourself and Oceiros if he takes to the air. He’ll pound the ground in just a moment. That move deals minor damage and interrupts whatever you’re doing, so you can’t just trade blows with him.
Oceiros also has poisonous breath, so sprint away if he breathes. This prevents you from taking poison damage throughout the fight, and it also helps to keep your vision clear. It’s hard to see anything in the cloud.
Stage II
For the remainder of the fight, Oceiros ditches his staff and goes for claw attacks and mad dashes. His blows become more frantic and more dangerous. The single-claw swipe isn’t too bad, and its timing is slightly similar to the staff bash in Stage I. Instead, it’s the charge that you have to fear the most. Oceiros races forward on all fours, clawing through anything that gets in his way. Dodge repeatedly until you get to the side and are out of harm’s way.
You still have to watch the tail, but that’s not too bad. Use it as a way to dodge closer to the boss and gain free hits.
When you can, harp on executing head attacks. They’re risky, but enough damage can stun Oceiros briefly, giving you a second to heal or attack as you see fit.
You get a Bonfire, as usual. This time, you also get to explore a small area behind the boss’ lair. It doesn’t lead to a new region (not directly, at least), but there are secrets to uncover.
Open the door behind Oceiros’ room and descend. At the end of the tunnel, you meet a Serpent-man, which is a lesser form of dragonkin. You will encounter many more of them on Archdragon Peak. This one is alone, and you can kill it quite easily.
The corpse on the floor teaches you the Path of the Dragon gesture. A chest behind the Serpent-man contains a Titanite Scale, as well. The wall behind the rear chest is a secret door, so slash the wall to open it.
You’ve finished the Consumed King’s Garden. You can return to Lothric Castle, and continue clearing it, or advance ahead into the Untended Graves, located behind the secret door in this room.