I asked a similar question some time back, and I think that you'll be ok to see the lights after the show. We were there last week, and I think Fantasmic was over at 7:30. We also went on a night w/ EMH at MGM which was fabulous. After 8:00, the park was a ghost town, and we got to spend all the time we wanted with the lights and walked right onto some rides - it had been VERY crowded earlier in the day. Both Fantasmic and Osborne lights are not to be missed if at all possible!
Good luck!