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>