If you are logged on to your computer as a limited user and oblivion is installed somewhere to which you don't have write access, such as the program files folder, then obmm will start up in a special limited user mode. You have access to the save manager, bsa browser and conflict detector, and can check/uncheck esps. You have no access to your omod collection, any of obmm's settings or any other features.
Running in this way completely removes your ability to install mods. If you want to install and use mods as a limited user then oblivion must be installed somewhere to which limited users have write access. C:\Games\oblivion is a common choice.