From publisher blurb:
Old-School Essentials is an adventure role-playing game of exploration, danger, monsters, and magic. The game is intentionally rules-light, putting the focus of play on imagination, improvisation, and fun. The rules are optimised for ease of use at the table, with meticulous attention to wording and layout clarity. What's more, Old-School Essentials is 100% compatible with the classic Basic/Expert game from 1981, meaning that decades of adventure are at your fingertips!
Core Rules
Old-School Essentials is a modular game where the core rules of the game are extended by additional "rules modules" which add content for different genres of fantastic adventure. This book contains the core rules required to play Old-School Essentials:
Character creation and advancement.
Dungeon, wilderness, and maritime adventuring.
Combat and hazards.
Magic and spell casting.
Note that this book is not a complete game! To make a complete rule set, just add rules modules to tailor the game to your needs. A selection of official rules modules are listed below, but it is also easy to build your own homebrew content on top of the Old-School Essentials core rules!