The problem I have with OSR games is that there aren't really any guidelines that say "this is a 1st level or 2nd level encounter."
If the OSR game holds true to the B/X AD&D conceit of 1 HD = 1d8; Then It's implied in the monster stat blocks.
All NPC and Monster HD are d8's. A Sword wielded by a PC does 1d8 damage. That's your basic encounter math: The old chainmail conceit of 1HD = 1"Hit".
How many 1d8 Sword hits can the monster take before it goes down? You then bounce that off of How much damage can the monster do to the PC's before than happens. And you have a ballpark idea if the "encounter" will be too much for the PC's or not.
You even have morale rules that might cause a fight to end even before all the baddies are killed...
...
Plus, I think part of it is the shock of seeing a character like "you start with 1-2 spells, +0 to hit, and 8 Hit Points." These players would balk at being so underpowered, compared to 5e (which is their only metric).
It is easy enough to house rule max HP +Con at first level on top of the -10 HP before death rule as well.
Instead of 1-2 spells; start them out with 3-4.
You can even give them the average HD +1 HP per level that 5e does, or even max HP per level as they level up.
And if you want to add PC options; there are good 3pp that have 'feats' for B/X style OSR games.
The B/X based OSR games are very moddable to taste.
I'm not sure. The ones I've tried (OSE, S&W) are still very challenging to balance, because "balance" seems antithetical to the concept of OSR games.
Having grown up with TSR-era AD&D, I don't know how any of my PCs survived to mid-levels. Did we all play differently then?
It's not that "Balance" is antithetical; it's that PC's are expected to run or avoid encounters if they get in over their head.
One thing I don't like about the "encounter guidelines" Wotc has implemented since 3e is that they have basically trained the player base to feel that they not only can, but
should win
every fight if they just stick with it.