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.
An XPS file represents page layout files that are based on XML Paper Specifications created by Microsoft. This format was developed by Microsoft as replacement of EMF file format and is similar to PDF file format, but uses XML in layout, appearance, and printing information of a document. It is, in fact, more justified to say that XPS is an attempt on PDF, but couldn't get enough popularity as owned by PDF for a number of reasons.
You can also convert OST into many other file formats. Please see the complete list below.