Story related and cannot be missed. After defeating Ornstein and Smough, talk tonthe princess in her chamber and she'll give the Lordvessel to you. This allows you to fast travel and you must place it on the shrine in Firelink Altar (fall down the hole in the building with the puddle of water in Firelink Shrine).
Ornstein and Smough is a very hectic fight as you'll be fighting them both at once. Ornstein is fast and is easier to dodge but Smough is slow and he hits very hard. Circle the room and keep then both on the screen at the same time as much as possible. Attack when you are given an opportunity. Ornstein will stagger if 500 points of damage are done to him quickly so use this to get away. Upon killing one of them, the other will receive their power. If you kill Smough first, then Ornstein will get super big and will have an AoE and grab attack, both of which are devastating. If Ornstein is killed first, then Smough will become bigger and he will do lightning damage and have an AoE. Whenever you see them charge up their AoE's, roll away twice. You'll receive the soul of whoever you kill last.