OSE is just a re-organisation of 1981's B/X (Basic/ Expert) D&D by Moldvay and Cook, with a splash of AD&D's character classes thrown in. One appeal to the system is that the rule books (and the adventures for it, by and large) are laid-out in a very helpfully logical and instantly clear fashion. Another plus is that a million resources from other fantasy TTRPGs are compatible with the rules with little-to-no conversion. If it uses the six stats, has AC, saves and hp, then it's compatible with OSE.
I've played it for the last five years or so, and it's a sturdy and rules-light system.
It is a touch vanilla compared to Mork Borg or Dungeon Crawl Classics, but it is extremely easy to supplement and replace bits of it. I'm running a game using OSE at the moment to simulate the video game, Dark Souls, which I love to bits and my players have never played. I've swapped out the magic system for a somewhat spicier and more varied one from publishers, The Lost Pages, and switched out a few systems for ones from the video game (souls rather than XP, death causes respawning at the last bonfire rested at with all one's unspent souls dropped at the site of death etc.). It seems to be working fairly well, and I think it's because OSE is so simple and robust: it's a useful chassis.
The game designers say this about OSE: https://necroticgnome.com/pages/about-old-school-essentials
I do not receive a commission on sales of OSE products 😉 I just like the game a lot.
