To clarify, "treego" is refering to an installation of "Opera for Linux" in the Crostini Linux container of a Chromebook.
I am refering to the more usual way to install Opera on a Chromebook - from the Google Play Store where a slightly different version of "Opera for Android" is offered. However, "Opera for ChromeOS" seems very poorly documented.

BTW I have found the main settings, only available as a gear in the "profile" dialogue, which is only available in part-screen mode because scrolling of the task-bar is inhibited in full-screen mode. There seems to be no setting to auto-hide the side bar so it constantly takes up screen real estate. So does the tab bar because it is not integrated into the widget bar.

@mikeyates
Or, for that matter, get into preferences/settings?
It seems to be missing off the bottom of the left side bar.

@treego
OK, but how to hide the task-bar?
There is no "Alt-F11" on a Chromebook and the equivalent keys do nothing to the special version of Android Opera for Chromebooks.

Any chance of this happening?

Also, please give us a separate forum and separate help pages because many Android tips don't work in ChromeOS.

It installs from the Android Playstore, as do most Android apps, with a "desktop customised" version. Better than Chrome in several ways, especially privacy - separate password vault, cookies reliably deleted, etc. However, we could do with a separate help page because many tweaks for Android simply don't work in ChromeOS.
In particular, I'd like to hide the side menu bar for more screen space .Opera say "Alt F11" but there are no F-keys on a Chromebook!

We need a way to discuss this separately, please!
I found a 2020 closed thread in "Windows" about it, in which someone said they didn't think anything but Chrome is possible on a Chromebook but, even then you could install Most Android apps. Also, Opera have their own webpage saying the Chromebook version is specially formatted "with laptop features". However, there doesn't seem to be a special Help Page and many tweaks suggested under Android simply don't work, nor those for Windows, Linux and iOS.
Specifically, I'd like to be able to edit my speed dial and to hide the menu bar.