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.
A file with DOTM extension represents template file created with Microsoft Word 2007 or higher. It is similar to the popular DOCX file format other than it retains the user defined settings for reuse in case of creating new documents. Such documents are more often used in offices where a standard template file is generated with settings like page information, margins, default layout and macros, and is used to create new documents from it when required. DOTM files, however, save macros, that are a series of commands in the form of recorded actions for automatic completion of a task. This helps save time in carrying out actions that are repeated in completion of a task.
You can also convert OST into many other file formats. Please see the complete list below.