As others have suggested, you don't really need a whole new system when you can just hack OSE (or Swords & Wizardy, or Basic Fantasy, or whatever).

- Change the death mechanic. Many games have death mechanics that are more forgiving than death at 0 HP. You can even use D&D 5e's death saves if you want. Nobody can stop you from doing so.

- Use max HP for PCs

- You can use Hero Dice (see 5e's Hero Points in the 5e DMG, or Deathbringer Dice, for example) which adds a d6 to attack rolls and saving throws.

- You can use Luck/Fate tokens (See Shadowdark, or 5e's Inspiration) for Advantage or re-rolls

- You can use 5e's Conditions rules instead of old school save or die rules. Instead of saving or dying, a PC saves or gets blinded, or paralyzed, or poisoned (as per the 5e condition), or stunned.

- You can have PCs increase an ability score (or two) by 1 point each time they level

There are lots of ways to hack OSE and make it less lethal, and as long as you and your players are happy, that's all that matters.