I seriously doubt Oblivion loads all .bsa files into memory at startup. Just looking at my own folder, the bsa's add up to 13.6 GB, and that's before they are unpacked. No way the Gamebryo engine could handle that, especially Oblivion's old version. Windows does perhaps do some intelligent precaching, but I wouldn't count on it. As a general rule, packaging replacers and mods that one is likely to uninstall as .bsas seems counterproductive, both because they could get overwritten and because it could complicate uninstallation unless it was prepped as part of a BAIN installation.