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.
As of December 2013, according to the ENWorld Hot Games tracker the OSR made up 2.16% of D&D discussion on the internet.
History
The OSR was made possible by the OGL and the relaxed issues with copyright that it allowed. Either Castles & Crusades, or the Old School Index and Resource Compilation (better known as OSRIC), is considered to be the earliest OSR games and it includes most Dungeons & Dragons retro-clones. Other games considered part of the OSR include Red Box Fantasy, Lamentations of the Flame Princess Weird Fantasy Role-Playing, and sometimes Monsters & Magic and Dungeon World.