The OpenXPS format is the same as the XPS format, except that OpenXPS has been standardized as an open format by Ecma International, a standards organization. The ".oxps" extension was adopted for OpenXPS, but it is not often used since the ".xps" extension has traditionally been used for XPS documents.
NOTE: When printing in Windows 8 and using the Microsoft XPS Document Writer print option, the default extension for the output file is ".oxps." It may make sense to change the extension back to ".xps" if you plan to email the file, since the XPS extension is more commonly used.