OST or Offline Storage Files represent the user's mailbox data in offline mode on the local machine upon registration with Exchange Server using Microsoft Outlook. It is automatically created on the first use of Microsoft Outlook upon connectivity with the server. Once the file is created, the data is synchronized with the email server so that it is available offline as well in case of disconnectivity from the email server.
Files with .DOC extension represent documents generated by Microsoft Word or other word processing documents in binary file format. The extension was initially used for plain text documentation on several different operating systems. It can contain several different type of data such as images, formatted as well as plain text, graphs, charts, embedded objects, links, pages, page formatting, print settings and a lot others.
You can also convert OST into many other file formats. Please see the complete list below.