The DITA-OT toolkit which comes with <oXygen/> is located in the {INSTALLATION_DIRECTORY}/frameworks/dita/DITA-OT
directory.
You can configure another DITA-OT toolkit directory for use in <oXygen/> To do this you must edit the transformation scenario that you are using and in the Parameters tab change the "dita.dir" parameter to your custom DITA-OT installation directory. Also in the Advanced tab (the Libraries button) you have to add:
the dost.jar
and resolver.jar
libraries as file paths that point to
the libraries from your custom DITA-OT installation directory
the installation directory of your custom DITA-OT and the
lib
subdirectory of that installation
directory as directory paths