Is it more possible your oscar crashed into the side of the side of the tank and knocked the thermometer off? I cant see a thermometer hitting a fish with enough force to cause an injury given water resistance etc. Might the oscar be outgrowing its home?
Melafix isnt going to do anything for the injury, but it might stop secondary infections. Plenty of water changes, clean water and a stressfree environment will aid your fishes recovery.