Hi i have to admit i feel ashamed by asking this, but it's been a while that i played oblivion on pc(finished the game on pc)
And i feel like RPGing, oblivion on pc.
So here's a question.

I have oblivion and the Oblivion_v1.2.0416English.exe
and shivering isles and the ShiveringIsles_v1.2.0416English.exe

whats the procedure?
i installed oblivion then patched, then shivering isles then tried to patch and it failed,
thanks

Best Procedure:
Oblivion
SI
SI Patch v1.2.0416

***

However since you already installed the Oblivion patch before SI, you should be fine.