OST (.ost) stands for Offline Storage Table, also known as offline storage files in MS Outlook. OST files are the cache copy of an Exchange mailbox generated on the local end to work when the server is unavailable. Therefore, making OST file allows users to work in server downtime conditions which can later be updated to the Exchange Server the next time connection to the network is established. Moreover, OST files are replica of users mailbox stored in Exchange Server Database (EDB file). Everytime the connection to the server is available, the two files get synchronized with each other.