Almost all modern browsers now use the URL bar as a multi-purpose search bar. Instead of only being able to type web addresses, you can typically also enter search queries. This works because browsers now analyse the text in the search bar and identify if it is a URL or not. If the text looks like a URL then it is treated as such and the browser tries to navigate to the site. Conversely, if the text doesn’t look like a URL then it is forwarded to a search engine.
One of the difficulties with designing a browser with this feature is setting a default search engine. There are many options available, however, not everyone likes the same search engines. The way that browsers have dealt with this issue is to select a default and then allow the user to choose to change it if they prefer.
The Opera browser on Android uses Google as its default search engine. This is likely a popular choice, given that Google is the most used search engine. Some users, however, will want to change Operas behaviour to use a different search engine. The default search engine can be set from within Opera’s in-app settings.
To be able to access Opera’s in-app settings, you first need to tap the Opera icon in the bottom right corner of the app.

Next, tap “Settings”, found at the bottom of the popup pane, to open the settings.

Once in Opera’s settings, scroll down to the “Search” subsection, roughly halfway down the settings list, then tap the first option “Default search engine”.

Once you’ve tapped “Default search engine”, a selection panel will pop-up from the bottom allowing you to choose which search engine you want to use as the default. Your options are Google, Yahoo, Bing, DuckDuckGo, Amazon, eBay, IMDb, and Wikipedia.
The first few options are standard search engines, while the last four are more suited to users who regularly want to perform searches specifically on those sites.

Angelica says
I hope one day Opera includes an option to add or modify a search engine
Fred says
No. I don’t want any of those, but it doesn’t matter opera doesnt hsve sesrch engine as an option within settings