An OST file (.ost) is an offline folder file in Microsoft Outlook. Offline folders make it possible for the user to work offline and then to synchronize changes with the Exchange server the next time they connect. The ability to work offline is useful in environments with limited or unreliable connectivity. Microsoft Outlook application automatically creates an OST file or offline folder file when you initially configure the program to connect to an email account. A copy of your mailbox is stored in the OST file, so you can easily access your email even if you are not connected to the internet.
If you want to convert your OST file to PST file then follow the blog.How to Convert an OST File to PST in Outlook 2007 which i have discussed here at the same blog.
Step1: close your Microsoft Outlook application then select My Computer from start menu.
Step2: Navigate to the C:\Documents and Settings\username\Local Settings\Application Data\Microsoft\Outlook directory if you are using Windows XP. Otherwise, Windows Vista and 7 users need to navigate to the C:\User\username\Local Settings\Application Data\Microsoft\Outlook directory.
Step3: search in outlook directory to find the file named Outlook.ost. And Drag-and-drop the “Outlook.ost” files to your desktop.
Step4: Copy the alternate OST file into the “Outlook” directory and rename the alternate OST file to “Outlook.ost.”
Step5: After completing the all above steps then open the Outlook application. Outlook will now automatically import the new OST data and treat it as the primary OST file.