just issue this command on any Debian/Ubuntu-based system:

wget -O - http://deb.opera.com/archive.key | apt-key add -

and everything will be fine! 😉 🐧 🆙 \m/

Resolving deb.opera.com (deb.opera.com)... 185.26.183.130
Connecting to deb.opera.com (deb.opera.com)|185.26.183.130|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 3152 (3.1K) [application/pgp-keys]
Saving to: ‘STDOUT’

100%[=============================================>] 3.08K --.-KB/s in 0s

2016-09-10 04:41:08 (25.3 MB/s) - written to stdout [3152/3152]

OK