Modern Opera (based on Chromium) doesn't seem to allow changing key bindings, and I seriously dislike it's built-in ones for switching tabs.
This extension binds custom keys to switch tabs, as works in Firefox and Chrome.
To use it, just clone this repo, enable Developer mode in Opera, then in the Extensions tab do "Load Unpacked Extension..." (the button) on the cloned directory.
Then set hotkeys in extensions menu.