Offline Storage Table (.OST) files are created by Outlook /Exchange Server that keeps offline record of Exchange Server emails stored in the client machine which are inaccessible when orphan or corrupted.
(At the moment of downtime Exchange Server, OST file is a useful resource for email client. On the cache mode, Outlook user can recover deleted or lost emails, images1, files, email properties (like To, From, CC, BCC, Subject, Date and Time) from them.)
Both OST files and PST files is part of MS Outlook, as you know that PST File is Personal Storage Data Table which store all personal E-mail and their attributes but OST files is created by MS Exchange Server that's keeps E-mail record on off-line mode in client machine. When Outlook user does work on off-line or server downtime, OST file play vital role to help in recovery of E-mail with OST File Repair tool.
You read more about individual freeware product:
OST File Viewer
It is a marvelous tool to quickly scan unreadable OST file and view including Emails, Contacts, Calendars, Tasks, and Journals.
OST File Locator
Tool to search out OST file with their properties and including details from multiple locations existing in the client machine.
OST to PST Converter
Magnificently convert unreadable offline Outlook OST file into readable PST file including entire email properties.
OST to PDF Converter
Successfully save multiple emails of Outlook OST file into PDF file format including emails, contacts, journal, task etc.