Good luck...

And a pretty good predictor for how the rest of your run is going to go. Will you buckle down and git gud even though the fight seems wildly unfair? Or will you throw dung piles over the door?

The two-boss combo => Ornstein and Smough? It can kind of go two ways:

• the first time you play DS, you'll get to that point in the game, and then the fact that your build is probably sub-optimal in many ways, combined with not really having had to fight two enemies with different movesets before, means that you're going to have a really bad time. Can't kite these boys out to fight them separately. Anor Londo isn't particularly great for grinding - you get a lot of souls, but the drops aren't great, so you're probably going to be going back a long way to get better stuff.
• every time you replay DS after beating it, you know these guys will be coming, and you'll have a decent build and decent weapons specifically for them. AL is about the point where a magic build really takes off and you'll finish the entire boss battle in the time it takes to cast crystal soulmass about three times, but there's plenty of viable builds for them. Last few times I've played it through, I've beaten them first time - it's a completely fair fight and it's not hard, exactly; just very unlike anything else you've had to do in the game.

Alas, defeating O&S means that you've pretty much finished the good bit of Dark Souls 1. It's a near flawless-game up until then, enough to rate it as one of the finest games of all time, and then they ran out of dev time. Duke's Archives is good, but brief. Londo Ruins is fine, but the boss is DPS race. Tomb of the Giants is ass. Lost Izalith is beyond ass; should have remained lost. Fortunately, the DLC is excellent and you can go and play that instead.