I recently ran into this and wanted to share, as I'm sure others may run into this as well.
As many of you may know, Windows RT 8.1 includes a version of Outlook 2013...so that you're not limited to just using the Windows Mail app...which is great! However, what many may run into is that the default location for storing your OST is C:\Users\<login>\AppData\Local\Microsoft\Outlook. Which, if you have the 32GB Surface (or equivalent) you probably don't want a big OST file eating up your space. In my case, I installed a 32GB SDHC in my Surface RT and wanted the OST to be there.
The issue is that you can only set where the OST file is created during Profile creation...not once you've setup the profile. If you try once the profile is setup, the option to Browse and select where the OST file goes is grayed out.
This KB article http://support.microsoft.com/kb/2752583 explains how to set where to put the OST during Profile creation and *is* also applicable to Outlook 2013 on Windows RT.