I'm just gonna link a bunch of places to check out if you want to deep dive into the OSR wormhole:
Dungeons & Possums's blog post is great if you specifically want to get into D&D retroclones, and the adventure recommendations & GMing advice is pretty well curated from what I can tell.
However it doesn't suggest any non-D&D rulesets that are generally considered OSR. Some suggestions:
- Maze Rats, a simple 2d6+stat game, the majority of the game and its world is implicit in random tables.
- Troika!, a game loosely based on the mechanics for Fighting Fantasy gamebooks, with a wild planescape-by-way-of-80s-UK-tabletop-games setting.
- Knave, designed as a framework for running material written for old-school D&D and its retroclones, generates similar numbers to those games with a lot fewer rules.
- Into the Odd, a rules-light game about recovering strange magic items beneath an industrial city. Embodies the OSR assumption that combat is generally a bad idea as you *will* get hurt, by skipping any to-hit roll.
One of the things that characterises a lot of OSR material, though, is that the game ruleset takes second place to the conversation at the table and internal logic of the game world. Much like a good game of Dungeon World, a good OSR session will often go quite a long time without a die roll, just using common-sense rulings.