Besides, I had to replace libffmpeg.so becuase youtube said, my Opera didn't support h.264, so I downloaded opera-ffmpeg-codecs-58.0.3029.81-1-x8664.pkg.tar.xz from repo.herecura.eu/herecura/x8664/... and now everything is all right!
Thank you so much for pointing that out! I am finally able to play h.264 videos with opera on my fedora machine!