There are thousands of mods out there, but not all of them work together. Users just need to follow these instructions and they will. Most mods will come with specific instructions as well. This is a pretty simple process as well though as it typically just involves downloading and extracting the files into Oblivion's game folder.
Oh, and try to use only one mod as a start, and then enable them one by one, maybe it's not steam, but some incompatibility between two mods that's causing your trouble. Those who don't want to use Vortex can always just download and install mods manually. I wouldn't worry, Valve is working on getting everything to work right, they already fixed most issues with Morrowind. forums and i was playing oblivion with the Lattamer Catgirl revised race mod. Try the other mod managers mentioned in the threads. When installing into your /.steam directory, the Oblivion Launcher greys out almost all options except for Support and Exit. Mods that add new followers to Skyrim number in the thousands from magic. I had the same problem with some of the tools when I used a No-CD fix. Unfortunately this means most external tools won't work properly (yet). Once you’ve done that, reload your game (you’ll need to close out of it completely). As it seems the Elder Scrolls games themselves work on steam, but the exe files have been changed a bit to remove the DRM. You can also access this menu through Steam by clicking on Library and selecting Tools, then choosing Oblivion Mod Manager from there.