Declare a namespace prefix for the namespace in your XSLT and then select
using that prefix:
<xsl:stylesheet ... xmlns:os="urn:schemas-microsoft-com:office:spreadsheet">
...
<xsl:for-each select="//os:Row">
...
</xsl:for-each>
...
</xsl:stylesheet>
This usually results in XPaths that are easy to read. However, XSLT/XPath tools generate the following, equivalent code:
<xsl:for-each select="//*[local-name()='Row' = and namespace-uri()='urn:schemas-microsoft-com:office:spreadsheet']">
...
</xsl:for-each>