.OST files are a product of Microsoft Outlook, and contain your offline e-mail data.
Working offline means you have access to your data in situations where a connection to your e-mail server cannot be established. To make this possible, your data must be saved locally on your workstation.
.OST files are a format used by Microsoft Outlook to save your offline data on your workstation. The next time you have a connection and are Outlook can communicate with the e-mail server, your data from the .OST file can be synchronized with what’s on the server.
