My point was the OSR style game doesn't use DCs the way a modern game does and the GM does a lot more... GMing.
Not really true. OSR style games just have less well-defined skill systems, with the result that each encounter has its own ad hoc generated skill system. You can see this in plenty of the modules of the day, with each room given guidelines for how to resolve fortune when the players attempt to do various things. There really is no more or less ad hoc GMing required when the players purpose something not covered by the guidelines, the difference is only where guidelines come from, how robust and transferable they are, and whether there tends to be a uniform approach.
If you read a published module from the era, you'll find lots of "there is X% chance to this as you cross the room" and "calculate the percentage chance of success as follows" and "force the PC to make a save versus wands/paralyzation/etc to avoid the trap" or "have the PC roll under his DEX on a D20 to succeed" or "Make the PC make a bend bars/lift gates check to succeed". That's all setting the DC very much like a modern game, it's just hidden under all the clutter and typically hand crafted for each scenario.
But in my practice with GMs, they develop tool kits and tend to apply answers uniformly simply out of habit if nothing else, at which point you could actually write down the rulings as rules and you'd have a table of DCs similar to the guidelines of modern systems.