So for some reason my Oblivion is missing a bunch of textures and meshes and I need to reinstall. The thing is - how do I reinstall the Steam version? I don't want any of my current mods to stay - I want a fresh start.

Right click on the game in Steam and select "Delete Local Content..." Then you should check your Program Files\Steam\steamapps\common\oblivion folder and remove any extra stuff you want removed, and remove your Oblivion.ini from My Games\Oblivion\ etc.

Then just go to your owned buy not installed games in Steam again and download and install it again.

Why are you sure you need to reinstall? Do you know what owns the missing files? It is rarely necessary to reinstall in order to fix missing resources issues.

Everything for SI was missing and a lot for Cyrodiil was as well.

Did you have the original BSAs in the data folder, all four of them, and did you have DLCShiveringIsles.esp activated?

Yep. But it's done reinstalling so it should be okay now.