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.
DOCM files are Microsoft Word 2007 or higher generated documents with the ability to run macros. It is similar to DOCX file format but the ability to run macros makes it different from DOCX. Like DOCX, DOCM files can be store text, images, tables, shapes, charts and other contents. The capability to run macros make it easy to save time by executing the series of commands in the form of recorded actions for automatic completion of a task. DOCM files can be opened and edited in Microsoft Word 2007 and above.
You can also convert OST into many other file formats. Please see the complete list below.