O&S, probably a dozen times. The only real hiccup I had in the game. The only boss I think the game has that truly requires a good bit of skill, and that merely knowing their routines doesn't make the fight that much easier.
Four Kings, Gwyn, Moonlight Butterfly, Bell Gargoyles, Taurus Demon, Capra Demon, Sif, Quaelag, Demon Centipede, Nito were all one shot deals for me.
I died a few times to Seath (bullshit deaths IMHO), a few times to Bed of Chaos, a bunch of times to Firesage Demon, intro boss a bunch while learning to play, Iron Golem two or three times (falling off edge...) Gaping Dragon a bunch.
Most bosses aren't too bad and don't really require a ton of skill providing your weapons are powered up, you have a good shield and realize that most of them it's best to stick right up close OR stand far far away and pelt them with magic. For several of them I had been watching Speed runs and seen some of the strategies used which made it easier for sure. Then again... the entirety of Dark Souls is SUBSTANTIALLY easier once the element of surprise is gone from it.