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.
Text files created with Markdown language dialects is saved with .MD or .MARKDOWN file extension. MD files are saved in plain text format that uses Markdown language which also includes inline text symbols, defining how a text can be formatted such as indentations, table formatting, fonts, and headers. MD files can be converted to HTML with a program called Markdown. Markdown language is released by John Gruber.
You can also convert OST into many other file formats. Please see the complete list below.