well there are extensive mods... some of the ones I am currently using are: QTP3 (this makes the game look alot better), MMM (many more monsters, thought that isn't the name of the mod), Frans, AWS (atmospherics), Night Sky, the official and unofficial patches and lots of other stuff. OOO is pretty good too. If you want to use MMM, Frans, and OOO together, you need FCOM and some time to get it running. See this site: FCOM: Convergence
and that will help you through. You will also need Oblivion Mod Manager, BOSS, and Wrye Bash. You will need at least OMM and BOSS in any case to get all the mods working. BOSS sorts the load order. OMM if you download the OMOD files instead of the .esp or .bsa files, well it seems to work better but not all mods are in the .omod format.
Honestly, OB4 is a really great game and there are literally thousands upon thousands of mods for it. A bunch of stuff for everyone.