Desktop
Open the Opera browser on your desktop computer.
To the right of the address bar, select Easy setup.
Scroll down and select Go to full browser settings.
Scroll down to “Search engine”, and use the dropdown next to “Search engine used in the address bar”.
From here, you can change your default search engine.
Android
Open the Opera browser on your Android device.
Tap the profile icon in the taskbar, then tap Settings.
In the “Search” section, select Default search engine.
From here, you can change your default search engine.
iOS
Open the Opera browser on your iPhone or iPad.
Open the browser menu (three lines) > Settings.
Scroll down and select Default Search Engine.
From here, you can change your default search engine.
Prior versions on GitHub.