OSRIC, short for Old School Reference and Index Compilation, describes itself as “a compilation of rules for old school-style fantasy gaming…intended to reproduce underlying rules used in the late 1970s to early 1980s”.Although OSRIC never refers to this directly for legal reasons, it is intended to reproduce the rules of the first edition of Advanced Dungeons & Dragons.
The purpose of OSRIC is to provide publishers with a tool to legally produce gaming materials compatible with the non-copyrightable aspects of the underlying rules set. Gaming materials described as OSRIC-compatible alert potential users those materials may be compatible with the rules of late 1970s and early 1980s Advanced Dungeons & Dragons. -wikipedia
MORE here: https://en.wikipedia.org/wiki/Dungeons_%26_Dragons_retro-clones
Here is one example
Dark Albion: The Rose War is a 275 pages long, OSR RPG campaign setting, set in a fantasy medieval England during the War of the Roses (15th century). Think England + sorcery + demons and fairies + some fantasy twists to the world. 80% of the book is system neutral, so can be used with most role-playing games. Nonetheless, this book is designed with OSR games in mind, and will run best with Fantastic Heroes & Witchery of course, as well as similar games such as Osric, Swords & Wizardry, etc., and also the 5th edition of the most popular fantasy RPG of course! Yet, if you intend to run an Albion game with FH&W, there is a 20 pages appendix at the end of the book that proposes 8 classes specifically designed for Dark Albion, plus rules and guidelines relative to the setting.