Old School Revival
The Old School Revival, Old School Renaissance, or simply OSR, is a movement among players of tabletop role-playing games (especially Dungeons & Dragons) that draws inspiration from the earliest days of tabletop RPGs in the 1970s.
Broadly, OSR games encourage a tonal fidelity to Dungeons & Dragons as it was played in the first decade of the game's existence—less emphasis on linear adventure plots and overarching metaplots and a greater emphasis on player agency. Frequently they are built around older rules systems made available by the Open Gaming License (OGL).
The OSR was made possible by Wizards of the Coast's release of the OGL in 2000, which allowed the free and unapproved use of large amounts of creative and rules mechanic material related to the Dungeons & Dragons game.
The OSR label includes most Dungeons & Dragons retro-clones; most OSR games are variants of either the 1974 original Dungeons & Dragons rules (OD&D)—such as Swords & Wizardry—or the 1981 Basic and Expert sets of Dungeons and Dragons (known as B/X, or Moldvay/Cook, after those sets' primary authors)—such as Labyrinth Lord, Basic Fantasy RPG, and Lamentations of the Flame Princess. Other entries include 2004's Castles & Crusades, or 2006's Old School Reference and Index Compilation (better known as OSRIC) along with Basic Fantasy RPG 
In addition to the development of internet platforms and printed rule books, other printed OSR products became widely available. In 2008, Matthew Finch (creator of OSRIC) released his free and influential "Quick Primer for Old School Gaming", which attempted to sum up the OSR aesthetic.  Print-on-demand sites such as Lulu and DriveThruRPG allowed authors to market periodicals, such as Fight On and many new adventure scenarios and game settings. These continue to be created and marketed, along with older, formerly out of print gaming products, via print-on-demand services.
Style of play
The general ethos of OSR-style play emphasizes spontaneous rulings from the referee, or Game Master, over set rules found in a book. The idea is for the players to engage with the fantasy as much as possible, and have the referee arbitrate the outcomes of their specific actions in real time. The idea of game balance is also de-emphasized in favor of a system which tests players skill and ingenuity in often strange or unfair situations. The players should expect to lose if they merely pit their numbers against the monsters, and should instead attempt to outwit or outmaneuver challenges placed in their way. Keeping maps comes highly recommended.
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- Finch, Matthew (2008). A Quick Primer for Old School Gaming. Swords & Wizardry. pp. 5–8.