Gameplay Improvement - OOO is a major redesign of thousands of aspects of the experience of Oblivion. It incorporates dozens of existing mods, and adds new dungeons, underwater life, new races, and brings back slave traders in Cyrodiil among other things.
Official page for this project
Status: Version 1.33 is out !
Note: Remember to recreate the Bashed Patch file if you are using TNR in combination of OOO.
Additional resources :
Known incompatibilities (MMM, FCOM, etc)