Yes - it's very easy, just follow the installation instructions and you are asked to install the library on whichever drive you choose. The main application must go on Macintosh HD though (or whatever your boot drive is called).
You can also move the STEAM directory at any point in the future too should you wish to.
Ok thanks Tim.
By precaution, i install all dvd on HD, like instructions. So, if i can move the Steam directory, my problem is resolve. I finished my installation, and i move it later. Finaly, it's very easy to install Omnisphère.
Rogles - On Mac the AU version is about 95MB - so not huge, guess VST is about the same
Scatman - you can simply drag and drop the STEAM folder anywhere you like and as long as you create an Alias (OSX) or Shortcut (Windows) to the new location where you've moved it to then everything will work fine.
I also found this the easiest way to install on to my mobile rig which uses an external FW drive - just copy the STEAM folder from my main DAW, use Disc 1 to install just the AU component, create an alias and away you go.