For those like me who want to get Ornstein's soul and the leo ring by killing Smough first, I thought I'd share how I eventually beat them.
Both are weak to fire. I believe that at least Smough is also weak to lightning, but I don't believe Ornstein is. After failing a few times with my +5 Lightning Iaito, I decided to upgrade to a +4 Quelaag's Furysword, which does fire damage, and scales with Dexterity and Humanity (the latter capping at 10 humanity), which worked the first time.
I got the Dark Wood Grain Ring by killing Shiva of the East's bodyguard, who will only appear upon joining the Forest Hunter covenant. Generally, I found it to be a better idea to stay nimble in this fight rather than block too much, so the ring allows for very quick movement up to 50% equip burden. I stacked up on some Havel and Black Iron gear for the poise and phys/lightning defence, and did the best I could manage.
Block as you're entering in case of Ornstein's attacks from across the room, then move around a lot, behind pillars where possible for a respite. As Quelaag's sword stacks with humanity, which I had farmed already from the rats in the depths, I tried to use it rather than Estus flasks to heal, so as to get a damage boost at the same time. I went in with 2 humanity and ended with 5.
Smough is most easily dealt with using pyromancy if it's available, and it really helps with the upgraded Ornstein as well. Chaos Fireballs did a lot of damage, whilst regular fireballs were nice to fall back on when it was 1v1. Rolling toward's Orstein when he was attacking seemed to work well for me, taking 2 - 3 strikes then rolling away again.
Good luck, and know that the feeling of relief when you beat these two is definately worth the frustration of getting there.