Here James M at Grognardia talks about the definition of Old School gaming. I seen the Old School feeling argument made before and I see where James is coming from in that part of his post. However both sides are missing the point. It not about feeling, it is about attitude that makes a campaign old school. This transcends rule systems and setting.
I am not saying that ANY setting and ANY rules system can be used for old school gaming. Some are indeed easier to use for old school than others. But the concepts outlined in the Old School Primer can be used with a pretty large selection of Games and Settings that are out there.
Part of this stems from my experiences. Superficially my experience seems typical of many who write or sell for the current Old School market. I started with Holmes D&D, played AD&D, switched to other game systems (Fantasy Hero, GURPS), tried D&D 3rd, and finally came back to the original material.
Understand while the rule system had changed, the fundamental way I ran my campaigns has not. Instead of ditching what I did in favor of whatever the rule system advocated I adapted the rule system to my campaign. It just I prefer a more detailed combat system from being an avid wargamer. Today I have a lot of fun writing and publishing stuff for everybody in the Old School market. I never left and it is finally good to have a group to share my stuff with and a market to sell to.