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 PPSM extension represent Macro-enabled Slide Show file format created with Microsoft PowerPoint 2007 or higher. Another similar file format is PPTM which differs in opening with Microsoft PowerPoint in editable format instead of running as Slide Show. When run as slide show, the PPSM file shows the presentation slides with contents intact in the slide show and is in read-only mode by default. PPSM files can still be edited in Microsoft PowerPoint by opening it in PowerPoint.
You can also convert OST into many other file formats. Please see the complete list below.