Are there any screen locking solutions for openbox?
The simplest is slock, the suckless screen locker.
You could combine this with xautolock if you wanted to automate it after a period of inactivity.
If you want something more "featurefull" you could install xscreensaver. Of course, gnome-screensaver is an option as well...
I use xlockmore
Install (in fedora):
sudo yum -y install xlockmore xlockmore-gtk
and I have a keybinding (Ctrl+Alt+l) in my rc.xml file that goes like this:
<keybind key="C-A-l">
<action name="Execute">
<execute>xlock -mousemotion +description -mode blank -bg black -fg grey30</execute>
</action>
</keybind>