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Challenge for the Opera team.
Cracked open Microsoft Edge this morning and noticed a border around my browser tab area, not as big as that of Opera One but a bit irritating to me, I prefer borderless.
Microsoft has this simple option in among its Appearance settings for Edge. Disabling it renders the screen borderless just like before:
I look forward to such a capability available in Opera One, if it becomes available I'll definitely consider switching back to it.
@watersotr GUI = Graphical User Interface.
@burnout426's directions worked fine for me, but if you're completely new to using the command prompt it may not work out straight away, there are a few Windows-specific conventions that help to be aware of there.
I'm with you regarding the security aspect. Though I was able to run through the steps to fall back to the pre Opera One version, that means I'm basically stuck, and I won't upgrade unless Opera specifically offers an option to preserve the previous versions GUI aspects which I and so many others were very comfortable with. 
@watersotr Perhaps the Opera team should be congratulated for achieving what I suspect is a milestone, if a dubious one, in software development. I think, perhaps, they allowed either a single "rock star" developer, or that person and a gaggle of devotees, to push for what they thought were slick, shiny design and usability changes that turned out to be not so slick and not as usable as they'd hoped.
Whether young or inexperienced or overconfident or a combination, it's entirely possible for people to hyperfocus on some singular goal in development and ultimately neglect the rest, in other words not seeing the forest for the trees, as they say. I think with Opera One they did just that, and however unclear the circumstances may be to mere users choices were made that transcended subjectivity and have proven objectively poor.
Speaking as a longtime IT guy with a background including web and database development, and one having worked with and shaken my head sadly in the aftermath of such people, well-intentioned at best and frightfully egotistical at worst, I can only hope Opera will step back, course correct, and work more closely with their user base to devise a solution that not only deploys the bleeding edge some of their aspirants crave, but also cater to their devoted longtime user base that's stuck with them until recently.
@watersotr Another option might be to use a different browser, whether Edge or Chrome or Firefox or some other, until you are able to get the situation with Opera straightened out.
I've done tech support for a couple decades out of the five I've been around so to hopefully clarify some of the helpful info related to @burnout426's reply, the steps described might be easier by opening a Command Prompt in Windows, and then from this page copying the commands and pasting them into the command prompt window, and then when ready hitting Enter to execute them.
For some context, the items in between percent symbols such as %LocalAppData%, %UserProfile%, etc., are sort of generic commands which when typed from your system will open those system folders specific to your Windows installation. These kinds of generic things are also known as environment variables which help software developers among others provide relatively generic means of referencing system-specific things.
That means, for example, %LocalAppData% when pasted into your Command Prompt and hitting Enter will open a Windows Explorer window showing the contents of your C:\Users\<your Windows username>\AppData\Local. The same would occur if you were to paste %LocalAppData% after clicking your Start button (or on older Windows versions into Start => Run).
When running commands there with things after a command to run an EXE file, like --launchopera=0, that serves to specify additional parameters that do other stuff available for typically very specific uses that program offers.
I don't want to derail you from burnout426's explanation in guiding you so please consider my info here as side stuff to hopefully help clear the mud. If there are any specific Windows questions feel free to ask here or separately, I'm familiar with Windows all the way back to 3.1, and MS-DOS as well, so I have a pretty holistic history with its nuances and quirks at the command prompt level.
@burnout426 Thank you for these steps, worked nicely for me to revert back to the previous version. I hope Opera reconsiders and provides the more bubbly, rather irritating Opera One style optional, I like to keep it up to date for security and core functionality otherwise.