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.
Files with .POTX extension represent Microsoft PowerPoint template presentations that are created with Microsoft PowerPoint 2007 and above. This format was created to replace the POT file format that is based on the binary file format and is supported with PowerPoint 97-2003. The files generated can be used to create presentations that have same layout and other settings required to be applied to new files. These settings can include styles, backgrounds, colour palette, fonts and defaults. Such files are generated in order to create ready-to-use template files for official use.
You can also convert OST into many other file formats. Please see the complete list below.