I was reading up on Battlesystem and when I noticed that they talk how it works with BECMI D&D. Playing around with it I see it working with Swords & Wizardry, Labyrinth Lord, and of course OSRIC. Can work with the D20 as long as you realize that THACO means whatever you need to hit a 20 AC.
If I can figure out the math behind the damage chart I bet I can make a retro clone out of it.
To me the Old School Renaissance is not about playing a particular set of rules in a particular way, the dungeon crawl. It is about going back to the roots of our hobby and seeing what we could do differently. What avenues were not explored because of the commercial and personal interests of the game designers of the time.
What are RPGs?
A game where the players play individual characters interacting with a setting with their actions adjudicated by a human referee.
Rules are an aide to help the referee adjudicate actions and to help the players interact with the setting.
Dice are used to inject uncertainty which make a tabletop RPG campaign more interesting than "Let's Pretend".
The only thing a player needs to do to roleplay a character is to act if he or she was really there in the setting in that situation.