Download or install chromium-codecs-ffmpeg-extra deb package (version should be close to Opera's chromium version)
# This version can be extracted like this for example
strings /usr/lib/x86_64-linux-gnu/opera/opera | grep -e 'Chrome/.\+'
Either:
cat /var/lib/dpkg/info/chromium-codecs-ffmpeg-extra.list
Or:
dpkg -x <package> <dir>
Might be useful
# Declare OPERAVERSION to major opera version number (32,33,34 and so on)
# Declare OPERADIR to /usr/lib/x86_64-linux-gnu/opera-developer,/usr/lib/x86_64-linux-gnu/opera-beta or /usr/lib/x86_64-linux-gnu/opera
sudo mkdir $OPERADIR/lib_extra
sudo cp libffmpeg.so $OPERADIR/lib_extra/libffmpeg.so.$OPERAVERSION