Here’s how it looks on my system (Debian/unstable):
In /usr/lib/x86_64-linux-gnu/opera-stable/, I have a folder named lib_extra:
/usr/lib/x86_64-linux-gnu/opera-stable/lib_extra/
In $HOME/.local/, I have a folder named opera:
$HOME/.local/lib/opera/
I always place the appropriate libffmpeg.so file in $HOME/.local/lib/opera/ (without root privileges).
Then I create a symlink from $HOME/.local/lib/opera/libffmpeg.so to /usr/lib/x86_64-linux-gnu/opera-stable/lib_extra/libffmpeg.so:
As root:
ln -s /home/[username]/.local/lib/opera/libffmpeg.so /usr/lib/x86_64-linux-gnu/opera-stable/lib_extra/libffmpeg.so
Currently, the libffmpeg.so file from the Vivaldi browser is included there.
As a user (without root privileges):
cp /var/opt/vivaldi/media-codecs-8.1/libffmpeg.so $HOME/.local/lib/opera/libffmpeg.so
It works for me without any problems.
Of course, you can copy the libffmpeg.so file directly—without the symlink—into the lib_extra folder:
cp /var/opt/vivaldi/media-codecs-8.1/libffmpeg.so /usr/lib/x86_64-linux-gnu/opera-stable/lib_extra/