On the right side of OBMM's program window, there should be a number of options including "Launch Oblivion", "Settings", "Help", and "Utilities". Click on utilities and select "Archive invalidation". Make sure "BSA redirection" is checked and click the "Update now" button. You should get a popup saying "done" and then...well, you're done. You never have to fuss with archive invalidation again regardless of which mods you decide to install in the future.