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I'm surprised at the lack of condescending comments. It's nice.
They are definitely one of the hardest bosses to solo. You're very far along on your path to mastering this game now that you've managed this. You also get to enjoy one of the perks of doing so; you can now summon people on your subsequent play-throughs with the knowledge that you don't NEED to. Congratulations!
I wouldn't say they're the hardest boss in the game necessarily, but certainly the largest jump in difficulty. They are supposed to be essentially the ultimate test after all, it'd be a failing if they were easy. As well, short of Four Kings, Pinwheel, and one DLC boss (kind of) it's the only "more than one enemy" boss. Four Kings people get mixed results on, Pinwheel is easy-pie, and the DLC is DLC anyway, confront it at your leisure.
Off-topicish: I forced myself to solo O&S. I'd been summoning for them (and other bosses) my first time through the game, but I was one of those Drake Sword/Lightning Spear Turtle guys so they kind of ruined my game. After I rage quit and came back decided I would either get become skilled enough to take them on solo, or just stop playing. So I got skilled, at least comparatively, and haven't looked back.
This is a unique signature. I've put it here, so hopefully when you see it again you'll think "Oh yeah, that guy with the stupid signature!"