The XMLPaper Specification (XPS) was originally created by Microsoft and then adopted by ECMA TC46 as ECMA-388, the Open XML Paper Specification.  This is also referred to as OpenXPS.  XPS has the file extension of XPS and OpenXPS uses the OXPS file extension. These two file formats aren’t the same. Windows Vista and Windows 7 both contain support to view and create XPS files.  You can choose to print to the “Microsoft XPS Document Writer”  (MXDW) to create an XPS file and use the XPS
to view them. 

Windows 8 can view and create both XPS and OXPS files.   See OpenXPS Support in Windows.  The “Microsoft XPS Document Writer”  defaults to creating an OXPS file.  When creating a document, you can choose which file type to create when you are prompted for the filename.  You can change this default using the group policy editor or use PowerShell as described in Changing the Default Format for Microsoft XPS Document Writer.

You can’t view an OXPS file on earlier versions of Windows (i.e Windows 7, Vista, or WinXP) because, these operating systems only support XPS.  If you try and open an OXPS file in the XPS Viewer, you will get the error “The XPS Viewer cannot open this
document”.  You can’t simply rename the OXPS file to XPS because, these formats aren’t exactly the same.  If you try this, you will get the same error.   These formats are compared in the OpenXPS Document Comparison Result (Open XPS Format) paper. 

The Microsoft XML Paper Specification Essentials Pack (version 1.2) provides support for Vista and Windows XP to view and create XPS files but, not OXPS files.

However, you can use the XpsConverter utility to convert OXPS files to the XPS file format.  The Windows Driver Kit 8 (WDK) which used to be called the DDK, describes the XpsConverter tool.  This tool isn’t included in WDK 7. 

XPS Converter (XpsConverter.exe) is a command-line tool for converting XML Paper Specification (XPS) documents from Microsoft XPS (MSXPS) to standardized OpenXPS, and from OpenXPS to Microsoft XPS (MSXPS).   When you install the WDK, the
XPSConverter.exe file is placed in the %programfiles%\Windows Kits\8.0\bin\<arch> or %programfiles(86)%\Windows Kits\8.0\bin\<arch> directories.

Here’s the syntax to convert an OXPS file to the XPS format:

XpsConverter /XPS /InputFile=sample.oxps  /OutputFile=sample.xps