The dirty little secret about those games is that you lose because you don't know ________.

"How to play"? I mean, yeah, that's generally the reason games are considered hard. It's not really a secret. Not understanding that healing while enemies are already charging an attack, not understanding when to block, dodge, parry, which attacks to use and when to use them, how to position yourself correctly, how to properly utilize all your equipment and magic and how to exploit openings in enemy attack patterns are the main reasons people find the game difficult. All of that can be figured out by staying defensive and paying attention.

Once you know it, it's not that bad, but because the checkpoints are spaced so far apart, you waste a ton of time between attempts to learn more about the game.

They're not that far apart. Depending on which boss you're fighting, it usually takes between 30 seconds to two minutes to sprint to the fog. Being able to get to a boss' fog with the maximum health and Estus is a minor but somewhat important part of the challenge, so it's not really a waste of time. Additionally, it's there to make you think about why died for a moment instead of instantly going back to the fight.

Trying beating O&S normally and you enter the room, you see about 15 seconds of them, you die, load back to the checkpoint, then you spend another minute running back, and loading in again, only to see another 15 seconds of them before you die. Lots of wasted time not spent on the boss fight, only learning a few seconds at a time.

Fifteen seconds? I'm sorry, that is a very, abnormally small amount of time. Maybe that could happen the first time you enter the fight if you get overwhelmed and then cornered because you forget to stay away from the walls, but even then that's your fault for not being as patient and defensive as you should be your first time in a boss. If this happened to you everytime I'm forced to assume that you didn't take the time between checkpoints to think about why you died if you only survived for a few seconds everytime.

Try turning on invulnerability and putting down the controller. You can just watch their attack pattern, learn their moves, and the opportunity windows for you to attack.

You're supposed to learn that stuff by playing defensively until you have an okay grasp of their moves. If you just block and walk backwards it's easy to stay alive and watch their attacks for a long time. There are a few bosses where this isn't the case, mainly Bed of Chaos and Capra Demon (which are almost universally seen as the least fair in the game), but Ornstein and Smough aren't like that.

Then you turn off invulnerability and then fight them normally, now armed with your observations. You'll end up beating those encounters in a fraction of the time, because a huge portion of beating of that game is just obtaining the knowledge necessary to win. That knowledge is just gated off through checkpoint travel time.

But you get that knowledge just as quickly from simply watching them with the mechanics provided in the game. You'll only die this way if you aren't paying attention to your surroundings and back into a wall.