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 XLTX extension represent Microsoft Excel Template files that are based on the Office OpenXML file format specifications. It is used to create a standard template file that can be utilized to generate XLSX files that exhibit the same settings as specified in the XLTX file.
You can also convert OST into many other file formats. Please see the complete list below.