DITA_OT_DIR refers to the default directory that is specified for your DITA Open Toolkit distribution in the Window > Preferences > Oxygen XML Developer plugin > DITA preferences page.

For example, if you are using DITA OT 2.5.2, [OXYGEN_INSTALL_DIR]/frameworks/dita/DITA-OT2.x (or if you are using DITA OT 1.8.5, the default DITA OT directory is: [OXYGEN_INSTALL_DIR]/frameworks/dita/DITA-OT). You can also specify a custom directory.