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.
PPS, PowerPoint Slide Show, files are created using Microsoft PowerPoint for Slide Show purpose. PPS file reading and creation is supported by Microsoft PowerPoint 97-2003. The more latest version of this file format is PPSX which is based on Office OpenXML standards. PPS files can still be read by latest versions of Microsoft PowerPoint, but newly created files can only be saved in PPSX file format. When a PPS file is shared with another user and opened, it starts as Powerpoint show unlike PPT file which opens in editable mode.
You can also convert OST into many other file formats. Please see the complete list below.