OST or Offline Storage Files represent user's mailbox data in offline mode on local machine upon registration with Exchange Server using Microsoft Outlook. It is automatically created on the first use of Microsoft Outlook upon connectivity with 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 email server.
Files with .EPUB extension are an e-book file format that provide a standard digital publication format for publishers and consumers. The format has been so common by now that it is supported by many e-readers and software applications. For example, on Mac OS, the pre-installed Books software provides the support for opening such files. In addition, there are a lot of compatible software available for smartphones, tablets and computers.
You can also convert OST into many other file formats. Please see the complete list below.