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.
Introduced and documented by Microsoft, the Rich Text Format (RTF) represents a method of encoding formatted text and graphics for use within applications. The format facilitates cross-platform document exchange with other Microsoft Products, thus serving the purpose of interoperability. This capability makes it a standard of data transfer between word processing software and, hence, contents can be transferred from one operating system to another without losing document formatting.
You can also convert OST into many other file formats. Please see the complete list below.