If it helps any, Smough is weak to fire, while Ornstein resists lightning. Personally, I find Smough easier to deal with after power-up than Ornstein, though.
Everyone does; Smough's power-up just increases his damage (and changes his resistances), while Ornstien's changes his move list and makes him harder to predict.
I have various things to say about Dark Souls...
First is that the part i found most annoying to play through was Sen's Fortress with a close second being the Library after Anor Londo, especially after the auto-death to Seath.
Second is that Blighttown is REALLY easy if you bring a bow and snipe stuff out of the way before you get there. Manual aiming on the bow absolutely ROCKS, and can even make you able to completely bypass all of New Londo by sniping the dude that's up the tower (forgot his name), and getting the Key that unlocks the path to the turn-thing, thereafter you have a new path opened up that's Firelink -> New Londo - Valley of Drakes (bridge part).
Third is that Pyromancy is completely and utter OP, and if you farm the souls you get in the dragon-guarded bridge when he breaths fire and kills all the Hollow Soldiers, you can then fast-track to Lower Undead Burg, kill Capra Demon, go to the Depths and free the pyromancy guy. Then, spend all your multiple-thousand souls into pyromancy upgrades, and go to Blighttown. In Blighttown, look for the woman that's Queelag's sister and teaches pyromancy, and farm souls until you get Pyromancy Gloves up to +15 and have bought all the spells. Shouldn't take long - Each trip up the bridge to the dragon gives you 555 Souls, and you can repeat that in about 15 seconds per run. After you've done that, you can power-through almost the entire game by just spamming Pyromancy that'll 1-hit-kill almost every mob, and even 1-hit-kill or 2-hit-kill almost all bosses with Power Within (which you get in Blighttown, and is just OP and amazing). You can up the damage of Pyromancy by quite a lot by farming Humanity, too. You simply go into the depths, and farm the rats, preferably using the Gold Serpent Ring from Sen's Fortress, until you go to about 30, which is when it stops scaling pyromancy IIRC. You don't even need a goddamn weapon, and can even beat the game at SL1 - as has been proven by multiple youtubers. But how about Queelag you say. She's immune to all pyromancy you say. Well, get a fast-firing bow, hundreds of arrows, and keep shooting at her female body. It has no poise and anything you hit said part with will poise break her and make the spider unable to move. There's also a safe spot in the area where you can do this from. Easily.
Fourth is that Magic is also completely OP, but not so much as Pyromancy, since you actually have to gain levels and stuff before magic really becomes an asset. You also need specialized gear, and Pyromancy doesn't. You can be anything and still have Pyromancy Maxed Out, whereas magic you need to spec for it. It's still very much powerful. In the beggining it's shit. But after you get Greater Soul Arrow and Greater Heavy Soul Arrow, both of which you can get just after Lower Undead Burg, and eventually Homing Soulmass and it's Greater version, which you can get after Sen's Fortress when you know what you're doing, it becomes insanely powerful.
So basically, Dark Souls is really only a difficult game the first or second time you play it, it then becomes a complete joke, because you can overlevel everything and anything, and you can always rely on pyromancy to be your go-to thing. What does it matter that stuff gets harder to kill in NG+ and beyond? You can still roll and dodge.
That's actually the 5th thing i wanted to talk about. Dodging. Dodging is really much, much better than armor, in 99,9% of the game. You can actually complete the game naked because there are NO situation where you're forced to tank a hit (1st Seath fight doesn't count since it's an auto-death either way - and that's why you procured a rare ring of sacrifice before then - That's also my biggest pet peeve with the game. NO GAME WHATSOEVER should force an auto-death). I don't like using Havel's Set for that reason, because it forces mid-rolling even with topped Endurance. It's always better to be less protected, but have the fast roll, than to have an awesome protection and mid-roll. You'll die much more if you aren't able to dodge. This is from experience... Only time it MAY better to wear armor is in PvP, if you use strength weapons and such, because then you can tank poise hits and still dish out the hurt.
Regarding O&S, yeah, Leo Ring beats Smough's Hammer any day of the week. Riposte is so strong after you get a really good piercing weapon, you don't even care anymore. Power Within + Red Tearstone Ring + Leo Ring + Riposte = One shot any parriable enemy in the game. You can actually one-shot Gwin with this i think. Power Within + Red Tearstone Ring is actually how people do 1-shot-all-bosses runs anyhow. Pyromancy gets ridiculous with this. Magic too, but not so much.
Anyway that's my final opinion on the game after dropping it for good.