About OST File
MS Outlook uses OST(Offline Storage Table) to store and view offline Outlook Exchange Servers files. There are mainly two scenarios when the Outlook's OST file communicates with the Exchange server:
First, with Microsoft Exchange Server's Cached Exchange Mode,
Second, with the Outlook Connector for Windows Live Hotmail.
Basically the OST file is the mirror image of the Exchange Server folders. And they only works with the conjunction with the Exchange Server during the synchronization.
The storage of OST file always resides in the backend.