Old-School Essentials: Author's Notes on the Rules [PDF]
Old-School Essentials is the product of over two years of poring over venerable tomes, discussing rules minutiae with the elite of online RPG geeks, laborious writing, and painstaking layout work.
The result—in the form of the Core Rules and Classic Fantasy books—is a game 100% compatible with the classic Basic/Expert rules from 1981. However, no game is perfect, and Old-School Essentials attempts to fix some of the most irksome errors in the Basic/Expert rules.
This document describes the tweaks that were made, for those with a passion for rules archaeology.