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 MHTML extension represent a web page archive format that can be created by a number of different applications. The format is known as archive format because it saves the web HTML code and associated resources in a single file. These resources include anything linked to the webpage such as images, applets, animations, audio files and so on. MHTML files can be opened in a variety of applications such as Internet Explorer and Microsoft Word.
You can also convert OST into many other file formats. Please see the complete list below.