OST or Offline Storage Files represent user's mailbox data in offline mode on local machine upon registration with Exchange Server using Microsoft Outlook. It is automatically created on the first use of Microsoft Outlook upon connectivity with 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 email server.
Files with PPTX extension are presentation files created with popular Microsoft PowerPoint application. Unlike the previous version of presentation file format PPT which was binary, the PPTX format is based on the Microsoft PowerPoint open XML presentation file format. A presentation file is a collection of slides where each slide can comprise of text, images, formatting, animations, and other media.
You can also convert OST into many other file formats. Please see the complete list below.