As it is discussed every second day, opera's built in libffmpeg does not work with h264 (and other) codecs. This is what mediainfo says about a random video I downloaded from bitchute a few minutes ago
Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Audio
ID : 2
Format : AAC
Format/Info : Advanced Audio Codec
Avc = h264 and aac is also not supported.
Since you are on an arch based distro, I suggest installing herecura's repo and installing opera ffmpeg codecs from there.