From the file's introduction: Advanced Microlite20 OSS (Old School Style) is a trimmed down, subminiature version of the Primary Fantasy SRD rules that has been designed to be quick and easy to play. The goal was to create a simpler game, but one where all of the resources of Primary Fantasy SRD (monsters, spells, adventures and equipment) could be used without conversion. Advanced Microlite20 OSS is a Microlite20 variant designed for old school style play with a modern game system and nearly any fantasy adventure module or setting you can pull off your shelf.