Help:Using Openbox in KDE
Starting Openbox with KDE
To log into the K Desktop Environment with Openbox as your window manager, select the "KDE/Openbox" option when logging in through KDM, which you can see in Figure 1.
If you don't use a graphical log in, you can use the
openbox-kde-session command to start a KDE session with Openbox as your window manager.
See the getting started guide for more details.