I've got a key named unofficial oblivion patch key, what is that for?

I think when you equip it, it opens a menu for you for certain settings. Just do it, no harm will be done. Normally you should not have to use it though. You can place it in a safe container until the time you do need it (which ideally is never).

Maybe read the opening post of the UOP thread? Not sure, but I guess they would mention it's purpose in there.

it can be used for a lot of things its called the unofficial oblivion patch and it can fix some problems in the game when you encounter them. You can fix your attribute stats if something is wrong with them, you can do some other stuff too try clicking on the key and a window will pop up asking what do you want to do and you will be given several choices