I receive a lot of questions from users who want to move the data files created by EAS (Outlook.com) and IMAP accounts. While you do not need to backup the .ost files, since they contain an offline replica of the mailbox on the server and can only be opened by the account that created the .ost, users with small boot drive may need to move the data files to a larger drive.
There is a registry value you can set that relocates the folder used by the .ost files. However, you need to create this registry value before you create the account in your profile. Editing the registry after the .ost is created will not move the data files used by existing accounts.
Create the ForceOSTPath string value in the registry and add the folder path as the data value, then create the account in your profile and Outlook will use this path for new .ost files.
Outlook 2013
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook
Expandable String Value: ForceOSTPath
Value data: full path you want to use
Outlook 2010
HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook
Expandable String Value: ForceOSTPath
Value data: full path you want to use
Outlook 2007
HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook
Expandable String Value: ForceOSTPath
Value data: full path you want to use
Outlook 2003
HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook
Expandable String Value: ForceOSTPath
Value data: full path you want to use
More Information
Moving an Outlook offline data file (*.ost)
How to move the IMAP personal folder (*.pst)
Published September 3, 2013. Last updated on September 3, 2013.