Default Openbox comes with keybinds for changing desktops using Ctrl+Alt+[Left|Right]. I prefer Alt+Number, because it's shorter, and then I know exactly which desktop I'm going to.
Add lines in ~/.config/openbox/rc.xml like below (and execute system/reconfigure openbox from the right-click-menu):
<keybind key="A-1">
<action name="Desktop">
<desktop>1</desktop>
</action>
</keybind>
<keybind key="A-2">
<action name="Desktop">
<desktop>2</desktop>
</action>
</keybind>
<keybind key="A-3">
<action name="Desktop">
<desktop>3</desktop>
</action>
</keybind>
<keybind key="A-4">
<action name="Desktop">
<desktop>4</desktop>
</action>
</keybind>
<keybind key="A-5">
<action name="Desktop">
<desktop>5</desktop>
</action>
</keybind>
<keybind key="A-6">
<action name="Desktop">
<desktop>6</desktop>
</action>
</keybind>
