Old-School Essentials (OSE) is a tabletop role-playing ruleset designed by Gavin Norman and published by Necrotic Gnome in 2019. Its initial release came in five books — Core Rules, Genre Rules, Cleric and Magic-User Spells, Monsters, and Treasures — and its aim was to reorganize the rules for clarity rather than to overhaul them.
OSE is built on the 1981 B/X (Basic/Expert) sets of Dungeons & Dragons, with the Basic set written by Tom Moldvay. It is a retroclone — a faithful, compatible reproduction that leaves the original rules unchanged while presenting the same system more clearly. A separate Advanced Fantasy line broadens the options by incorporating material from Advanced Dungeons & Dragons.
OSE is regarded as a poster child of the Old School Renaissance (OSR), the movement that revives the feel of early D&D. Critics praised its organization and clarity, and it is cited as one of the most highly recommended D&D retroclones.