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.
PPSX, Power Point Slide Show, file are created using Microsoft PowerPoint 2007 and above for Slide Show purpose. It is an update to the PPS file format that was supported by Microsoft PowerPoint 97-2003 versions. When a PPSX file is shared with another user and opened, it starts as PowerPoint show unlike PPTX file that opens in editable mode. The sequence of slide show is the same as in the original presentation. All the slides accompany the images, sounds and other embedded media accompany the presentation slides to the PPSX during the slideshow.
You can also convert OST into many other file formats. Please see the complete list below.