OST file is one of the database files used by Microsoft Outlook. Its main purpose is to serve as an enabler for working offline even when the connection to Exchange is not live. In such scenarios, the changes you make are later synced to the Exchange Server when the connection is back live.
If there are any errors or corruption issues in the OST file, the information stored within it comes under risk. This is mainly due to the fact that OST files, unlike PST files, cannot be opened independently of the Outlook profile that created them.