As Patrick Stuart notes, BX D&D is core to the OSR. It has clarity, it has concision, and it is very clearly “D&D”. It has a clear modern presentation in Old School Essentials. Sadly, it has some serious problems. Here are some fixes. Maybe later there will be more.
First, up the amount of gold needed to advance in level is absurd, unless you want advancement to be ponderous. I’ve mostly concerned myself with how to meet that demand, but if you read my articles about it you’ll find links that suggest other solutions. (the comments on those links are often good).
Specifically, look at the intro to Tomb of the Serpent Kings — “Treasure amounts are balanced around the idea that 200gp is enough to level a single character”. That’s likely to be my model going forwards.
If you’re not writing adventures for others, I’d suggest you (a) give more than one xp per gp or (b) rescale all the level charts so they give the pace you want.
It’s not clear at all, in OSE or otherwise, how thief skills relate to the (obvious) ability of non-thieves to do those same things. James V West has about twelve ways to fix that — start reading at http://doomslakers.blogspot.com/2019/01/once-again-with-thieves.html.
Requiring “Read Magic” to use scrolls is excessive. Give Read Magic as an unlimited-use ability to all Magic Users — http://doomslakers.blogspot.com/2020/03/read-that-magic.html.