...until Opera fixes the security problem for those of us who wish to stick with one of the older versions I am not interested, though I liked and trusted Opera more than any of the other browsers. ...

It is useful for all browser users to have a clear understanding of one key reality: no browser maker supports their old browser versions with security updates for any major length of time.

Browsers in this era are universally 'free', but the writing and testing of browser code costs significant amounts of time and money. Each browser version released by a maker contains code that varies to some degree from their earlier versions, sometimes more extensively than other times. Every security update involves repairs or modifications of the relevant browser code... the more numerous old browser versions (and their unique internal coding) being "supported" by a browser maker, the greater the number of code repairs and retests that they must simultaneously perform as security flaws continue being uncovered over time. Supporting multiple old versions with security updating quickly becomes untenable for a 'free' browser from a business standpoint.

This is made even more significant for all the chromium-based browsers sharing the common chromium rendering engine code being developed and supported by the Chromium Consortium (such as Opera, Vivaldi, Edge, etc). That underlying engine code is continually being updated by the Consortium independently of the browser makers, with security updates typically emerging every couple of weeks and feature updates somewhat less often. As a result, the browser makers find themselves having to also update/retest their own added user-interface code (which provides the unique characteristics of each browser brand) typically every couple of weeks, just to keep pace with chromium security updates. Since each chromium update must be integrated into each browser version's design, the resulting code recompiled, and all of it functionally retested before releasing revised browser code, maintaining each updated design consumes significant time and money - multiplied by the number of versions being supported by the browser brand.

Moreover, occasionally chromium code may be altered by the Consortium in such a way as to be incompatible with key aspects of earlier chromium releases - which in turn can mean the new chromium code won't even work with a browser-maker's particular coding within an old version, forcing either version obsolescence or major code rewrite by the browser maker for that specific old version.

The digital world changes quickly and constantly. So does the digital security threat arena. Hence, it's wise to keep one's public-facing software 'current' with regard to the universe of software security threats and patches. Unfortunately, that also means accepting the reality that old software versions will be abandoned or obsoleted by their makers for sound business reasons. This is all especially true for web browsers, since by nature they constantly face the public Internet but are made available for free by their makers, hence costs are high and recovery is difficult. Berating a browser maker for not supporting old versions with ongoing security updates is not reasonable in the real digital world.

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.

Or they can use Opera GX.

but until Opera fixes the security problem for those of us who wish to stick with one of the older versions I am not interested, though I liked and trusted Opera more than any of the other browsers. On top of that, I have no way of knowing which version I had and if I were to go back I would want that specific version because I had figured it all out...

as you can problem tell, I am slow when it comes to tech stuff, easily frustrated.

on the other hand I have survived living in caves, have worked with street gangs and addicts, directed a substance abuse program, mined for gemstones all over this nation, cut and polished gemstones set them and did repairs in both gold and sterling silver, as well as fabricating new pieces, and have cooked in restaurants. I used those talents to put myself through school, earning an N.D. degree, and to support my work in the health care field for over 35 years, mostly at free clinics, etc etc.

I taught myself to use computers (without ever playing a game on one) about 30 years ago and cannot understand the lingo or keep up with any of the modernization.

@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.

what is GUI?
your directions about disabling Opera so it cannot force me back to the new version later - those directions did not work. and besides as others have said, that would mean my opera is putting me at risk because there would be no security updates.

Opera blew it with this new version.

none of that was on the Opera full screen page. no way at all off that except to use Task Manager to shut it all down... so I lost everything I had been working on in other tabs...

yeah. I tried to follow the directions to do all of that.
now I have given up on Opera and went back to Firefox and Brave because I will never do Edge or any Microsoft program if I can help it.

And, yes, if you block updates then you will no longer get security updates unless you do them manually.

Indeed. I tried to follow the directions for going back but sadly my many years on this earth and my declining ability to follow (and understand) directions, I gave up in the end and simply uninstalled Opera altogether. On my Surface I've reverted to Edge. On my MacBook I've reverted to Safari. I fear that my many years with Opera have finally come to an end. I noted your words "frightfully egotistical at worst" and thought that perhaps that might explain this new direction. It's simply not for me. I'll watch from the sidelines.

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.

all of the various explanations from word one.
so very complicated. impossible to follow.

I just want the old Opera which was easy to use. but I have no way of knowing which version I had before.

Too bad, Opera used to be great. Now in attempts to give more it has become far too complicated and specialized. You guys have lost a lot of users...

and all the supposed explanations about how to go back to the former Opera, all of them were super complicated and quite impossible to follow.

in fact, I am not even certain theere written in English.