By default, openbox assigns gnome-screenshot to print and gnome-screenshot -w (capture window) to alt+print.
Code:
<keybind key="Print">
<action name="Execute">
<command>gnome-screenshot</command>
</action>
</keybind>
<keybind key="A-Print">
<action name="Execute">
<command>gnome-screenshot -w</command>
</action>
So you can either install gnome-screenshot or change the above keybind in ~/.config/openbox/rc.xml so as to make it work with scrot.