Really looking forward to the first stable release of Vivaldi. There isn't any simple solution for linux based OSes.
Why not ship the needed library with opera or with an additional package (opera-codecs-ffmpeg-extra) or why not wait until Chromium update theirs?
I think there is legal issue in the US territory and Opera cannot afford to pay licence fees in the free product.
I would rather see smart post-install script (core deb and rpm functionality) that would symlink best found library to the opera installation (or leave embedded one).
Another option is using debian alternatives (configure with [update-alternatives --config opera-ffmpeg] or compatible gui)
Yet another option is to use installation time configuration (or change path with [dpkg-reconfigure opera-stable])
There is plenty solutions, but I guess not enough money coming from *nix, huh?
A don't have any time right now, but I will try to hack some solution using DPkg::Post-Invoke later on.
See you