You should find that option in User Interface settings tab.
But tbh I think is a pretty useless option, as mentioned above, it's the exact same thing as searching in the address bar, so why would you have two search bars next to each other that do the exact same thing? Plus the search bar is also in the speed-dial page.
The Speed-Dial page is used by most of the users to have quick access to the most used web pages and links without having to resort to bookmarks, and usually just so you don't have to write "youtube" "gmail" and other sites you visit very often in the URL bar.
Therefore I agree with you @wickerman18 , having to access it with a keyboard shortcut or enabling it on the sidebar settings is not very user-friendly. that's all.
I also hope they put the option to reactivate it,
especially for new users who may be disappointed by the absence of a home button and then go back to a previous version or worse switch to another browser.
But don't get me wrong, I'm loving Opera One and its new design,
as far as I'm concerned these are only very small details.