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.
ODT files are type of documents created with word processing applications that are based on OpenDocument Text File format. These are created with word processor applications such as free OpenOffice Writer and can hold content such as text, images, objects and styles. The ODT file is to Writer word processor what the DOCX is to Microsoft Word. Several applications including Google Docs and Google's web-based word processor included with Google Drive can open the ODT files for editing.
You can also convert OST into many other file formats. Please see the complete list below.