It's my opinion that O&S are the hardest boss fight in all the souls games. They probably aren't actually the hardest, but for the position they're at in the game I think they are. At that point you're into the game, but you haven't mastered it. It's also hard to travel around to get upgrades, because they're the thing gating teleportation.
All the other games that may have bosses that are more challenging, they're later in the game and/or optional. You'll also have more options available.
I think DS1 is one of their best games, but O&S are a bit of a mistake I think.