Osbourne Lights aren't a show of any type -- it's a walk-through attraction. The lights pretty much go on at dusk and stay on until the park closes. So it's not like Fantasmic is "before" or "after" the lights.
Depending on when Fantasmic ends up being scheduled (since Dec hours aren't out yet), you can see the Lights and then head to Fantasmic, or you can see Fantasmic and then head to the Lights. If you want to do it all in one evening, look for a night that MGM is open a bit later or a night when there are two Fantasmic shows scheduled. It will give you more time.