When Oxygen is installed in the Program Files folder, Windows requires administrative privileges in order to modify any file in there.
So you would have several options:
1) Start the application you want to use to edit the XSLT (Oxygen for example) with administrative privileges. This is usually done by right clicking its executable file and choosing "Run as administrator".
2) Install Oxygen in a folder where you have full access rights for the current user and use it from there.
3) Copy the entire DITA Open Toolkit folder:OXYGEN_INSTALL_DIR\frameworks\dita\DITA-OT
to a location where you have full access rights, make the modification there and then use that external DITA OT folder for publishing:http://www.oxygenxml.com/doc/ug-oxygen/#tasks/use-new-dita-ot.html
4) Perform option (3) but instead of directly modifying the XSLT stylesheet create a DITA Open Toolkit plugin which adds an XSLT customization for the XHTML output like:http://blog.oxygenxml.com/2013/12/creating-simple-dita-open-toolkit.html
Hope this helps,