In Linux Mint, Opera take the libpepflashplayer.so from /opt/google/chrome/PepperFlash. Try installing pepperflashplugin-nonfree doing sudo apt-get install pepperflashplugin-nonfree. It installs on /usr/lib/pepperflashplugin-nonfree. Then take the newly-installed libpepflashplayer.so and replace the Chrome one.

After doing that, flash worked OK on Opera.