In my case to enable DRM in Opera I had to create widevine_config.json file in /usr/lib/x86_64-linux-gnu/opera/resources directory.
widevine_config.json:
[ { "preload" : "/opt/google/chrome/WidevineCdm/_platform_specific/linux_x64/libwidevinecdm.so" } ]
I restarted Opera and live Youtube videos are playing at last!