I had the same problem recently when I got my camera. I do everything through VNC (that way only one keyboard/mouse on my small desk).
What you can do is from within the Pi VNC session
- From the menu top right select "Options"
- Open the "Troubleshooting" menu
- Check "Enable experimental direct capture mode"
The session will flicker, if it doesn't close the session and reopen.
You will notice the mouse cursor follows a dot, which is a bit annoying to begin with but you get used to it.
OMX should now work.
The main limitation here is framerate, its very slow compared to HDMI, particularly with OMX. When I'm doing camera previews I change the resolution down to 640x480 and the window size accordingly. Then if you're doing anything that requires higher framerates I connect back to HDMI.