My Data folder is in dire need of some cleaning. The plan is to remove as much as possible and reinstall mods with BAIN "install missing" feature. I just need to know what a vanilla (no mods) Data folder contains so I don't remove files belonging to Oblivion itself.

Best bet is to just reinstall Oblivion somewhere else .

Yep, I'd recommend reinstalling Oblivion too. One other thing you can do to keep things tidy: merge all of the Unoffiicial Patch meshes, textures, etc. into the vanilla Oblivion / SI / etc. BSA files using a program like BSA Commander. That will save you some disk space, make the game perform ever so slightly faster, and mean quite a bit fewer loose files in your Data folder. Just be careful - it's easy to mess up BSAs if you don't know what you're doing.

http://tesnexus.com/downloads/file.php?id=20258

Thanks for the advice! I'll try this OIC tool first.