OSR stands for "Old School Renaissance" or "Old School Rules" in role-playing games--referring to players returning to the original version of D&D published in 1974. Often designated as "OE", later editions edited by authors other than Gary Gygax and Dave Arneson would extend and enhance the game.

OSR Resources

So what's the appeal of OSR's take on RPGs? Well for one thing, this:

Basically, you can chalk it up to an era where the game was wide open, wild, and wooly. In it's heyday, the original fantasy roleplaying game thrived on the feverish imagination of pulp fantasy.