@blackbird71 Opening/closing has nothing to do with the creation of these files. I booted up about three hours ago and then launched my Opera browser, I did not open and close the browser multiple times, I opened it then started surfing the Internet. in three hours Opera created three of these files:
03/29/2018 04:44 AM 1,857,024 Opera_installer_180329084401885.dll
03/29/2018 05:06 AM 1,857,024 Opera_installer_180329090644175.dll
03/29/2018 06:12 AM 1,857,024 Opera_installer_180329101254337.dll

I must respectfully disagree with the characterization of Opera startup 'having nothing to do with the creation of these files'. While there may indeed be other mechanisms for creating new such files by a constantly-open Opera, on my system they are auto-magically created each and every time I start Opera - even if I start Opera, immediately close it, and then restart it again all within a minute or less. Each startup operation causes a file to be immediately written, differing only in the timestamp. Combining what you've observed with what I've seen, it's always possible that there's something in Opera that is writing a file at each startup, then periodically writing another file at some interval or operating event if Opera remains open.

Do have the problem since the last three updates.
Files are growing in c:\windows\temp and named like Opera_installer_180329212644142.dll, Opera_installer_180329212825812.dll, Opera_installer_180329213651953.dll, ...

Seems to be files generated when opera checks for updates.
It looks definitely like a bug and is eating up harddisc space.

Perhaps it's just another of the many mysteries involving programs running under Windows 10...

Could happen with programs, written in 1995 or so. Should never happen with current software. Windows 10 is not to blame, at least not in this regard.

@dede6 Those files aren't unuseful, they are created when Opera is installed or updated and should be removed (by Windows?) after that but it isn't.
Every time you install or update a software, it will write temporary files to the %temp% folder.

First, no third-party application should write into windows\temp folder - this is used by some windows programs (win defender, click-to-run telemetry) and/or in cases, when some non-interactive account (without folder in Users) requires temporary space. windows\temp is not %temp%. Looking at file security it seems that these files are created by system account - would it not our beloved Opera, I would ban such behavior.

Second, I see these dll files created multiple times each day (after starting opera), those are identical byte-by-byte (up to next real update), only creation time (and file name, which are correlated) differs by few hundreds milliseconds. This is clearly an error from opera updater side. Dev stream here.

Waiting for fixes. Patiently.

... Hmm that weird that Opera would generate them on startup and not just when installing/updating. What's your OS locale? Maybe if it's not en-US, Opera generates them automatically on startup for some reason. ...

I'm in the US midwest. About all I know is that this has been going on for some time with regard to both Opera and Windows versions on this 9-month-old desktop system, under both the original 1703 and 1709-upgrade Windows 10 Pro versions of OS. The older-dated files have been deleted during a couple of clean-up efforts, so I don't know the exact starting point in time... I do know that the last round of cleanup involved file dates starting around late January. Opera is set to (and does) allow auto-updating and those have been successfully occurring. Otherwise, Opera and my other browsers all run fine and uneventfully. Perhaps it's just another of the many mysteries involving programs running under Windows 10...

In looking into these files themselves with Wordpad, the contents below the half-way point include a great deal of plain-text in various languages that each appear to replicate the various text of Opera's installer-panel options,

Hmm that weird that Opera would generate them on startup and not just when installing/updating. What's your OS locale? Maybe if it's not en-US, Opera generates them automatically on startup for some reason.

For me, it's not that big of a problem to occasionally go in and manually scrub the temp folder of these files

Same here. Although, I think Opera could do some cleanup here just to be neat.

As in 'delete the \program files\opera\51.0.2830.55 folder' and not by "uninstalling" Opera?

The whole "opera" folder. You don't need to run an uninstall, but you can if you want to make sure the program and features entry for Opera is cleared and recreated when you reinstall.

I may be slow to respond here over the next few days.

Understood. No problem.

...
Can you delete the Opera program files directory and install again (with the offline installer just for good measure) and see if it still does it?

As in 'delete the \program files\opera\51.0.2830.55 folder' and not by "uninstalling" Opera? I'd prefer not to screw things up by doing it wrong.

By the way, due to a family medical crisis, I may be slow to respond here over the next few days.

with the offline installer

with the offline installer ya get lottsa o files in this Temp director too..

• Users\user's name\AppData\Local\Temp\Opera Installer -
  this is a sum of files but lottsa more o log files, to many to count lol

the exe files work and will re-install opera if ya click 'em..

On my system, an Opera_installer_yymmddtttt####.dll file is created and saved in the temp folder each time Opera is restarted

Can you delete the Opera program files directory and install again (with the offline installer just for good measure) and see if it still does it?

But in my case, files are created each time Opera starts, even if it is up to date.

I can't reproduce that.

And sometime, one file is created, but other times it can create several in few seconds.

The installer sometimes creates 4 - 6 of them for example. Looking at the log files in the "opera installer" folder in the temp folder, I see that sometimes one of the logs tries something and fails, which may be while it tries again. But, the other logs show that things were fine, so I'm not sure.

What's the path to Opera's launcher.exe? Is it just "C:\Program Files\Opera\launcher.exe"?

Since I often use multiple browsers via opening and closing each rather frequently (on occasion, multiple times each day), the number of Opera files in the temp folder can increase dramatically over time. Each file is 2.1 mB, so it does eventually add up unless they're cleaned out periodically.

Closing/restarting Opera immediately causes another file to be created - that is, there seems to be no time delay or limiting... a new, additional file appears every single time the browser is started.

I'm okay with temporary files for any soft that need to be updated or is updating.

But in my case, files are created each time Opera starts, even if it is up to date.

And sometime, one file is created, but other times it can create several in few seconds... And all the files are strictly the same. This point doesn't seem to be normal at me (I have computer skills, that's why I am surprised by this operation).