This is an optional boss, and is not required to reach the end of the game.
Once you have defeated The Dancer of the Boreal Valley, a ladder will fall down near the front of the room. Climb the ladder and make a left up the stairs, and take the Elevator down.
The water in this room will give you a poison effect if you stand in it for too long, so be quick!
Climb the stone structure at the end, and go up the elevator. After getting off the Elevator, turn right towards a large building. Kill the two knights, descend the stairs, and you will be at the boss fight.
Oceiros is not too hard, but his frost attack can really mess you up, so try to stay at his sides and back.
Once Oceiros gets to about 60% health, he will drop his weapons and begin to charge you.
When he charges you, dodge roll out of the way, hit his side a few times, and run.
Keep this up, and he will be dead in no-time.
Remember to light the bonfire, as shortly ahead is another boss fight: Champion Gundyr, which is just a buffed version of Iudex from the very beginning.
Hope this helps.
