Since you're in "program files (x86)", have you tried looking for the file in the virtual store folder somewhere inside your User folder?
Just found mine in "/Users/<xxx>/AppData/Local/Virtual Store/Program Files (x86)/" and low and behold a significant part of Blender was installed into there instead of into the 'real' program files folder.
(Guess me and my UAC will need to have a talk next!)
That's where ever since Vista the newer Windows OS make 3rd party apps put and access files into and from in order to 'protect' your program files folder from programmatic modification, whether you authorized this modification or not.
I'm not sure it will mess with the CS itself already, but any mod manager or even BOSS might end up moving your ESPs out of your program files and into this folder instead.
To make things worse, the Explorer will still show files where they are supposed to be, while any other means to locate them will not show them inside your program files folder anymore, as physically they aren't there but elsewhere.
It's actually 'lying' to you, making you think everything's alright, while in reality things are everything but, and your files are all safely stored far away from where the game can ever find them. (I hate OS that do that, lying to the user.)
That's why it's always the first advice 'not' to install Oblivion into the default location, somewhere inside program files, or any similarly protected system folder.
(I've actually had people complain in the past "But I already moved it to my Desktop!" Well, guess what? That's no less protected territory than any other part of Windows either, of course.)
If you find it where I pointed you to, and this keeps happening to you in future as well, it may be time to consider a re-install of Oblivion 'outside' of Windows' folders to stay clear of this UAC interruption once and for all.