BSA files that ship with the game as it can sometimes get confused and use those original files if it isn't done correctly. The game needs to have this done explicitly via the various methods outlined so that it knows to use the new textures as opposed to the default ones contained in the.

Read this link: Oblivion Mods FAQ - Archive Invalidationīasically, you use OBMM to cause the game's engine to not use the standard data files and have it use your replacement textures (or whatever other replacer mods you're using) instead.