@steaua10 Cookies can't be used in different installations, for the rest you can use Sync.

You could try this:

Install Chrome. Then, close it and Vivaldi.

Delete everything in the "C:\Users\yourusername\AppData\Local\Google\Chrome\User Data" folder (or move everything in there to a safe place if you actually use Chrome and empty the folder and move it all back when you're done doing all of this).

Copy everything in "C:\Users\yourusername\AppData\Local\Vivaldi\User Data" to "C:\Users\yourusername\AppData\Local\Google\Chrome\User Data".

Start Opera, goto the URL ```opera://settings/importData" and choose to import from Chrome to see if things work.

If the above doesn't work, you can try this manual method:

Delete everything in "C:\Users\yourusername\AppData\Opera Software\Opera Stable" and "C:\Users\yourusername\AppData\Local\Opera Software\Opera Stable".

Copy everything in "C:\Users\yourusername\AppData\Local\Vivaldi\User Data\Default" to "C:\Users\yourusername\AppData\Opera Software\Opera Stable".

Copy everything in "C:\Users\yourusername\AppData\Local\Vivaldi\User Data" (except the "Default" folder) to "C:\Users\yourusername\AppData\Opera Software\Opera Stable".

Move the "Cache" folder in "C:\Users\yourusername\AppData\Opera Software\Opera Stable" to "C:\Users\yourusername\AppData\Local\Opera Software\Opera Stable".

Then, when you start Opera, if you're lucky, Opera will just use everything from Vivaldi it can.

(Always test with copies of everything in your Vivaldi profile so that if things go wrong, you can try again, and so you don't mess up your Vivaldi profile.)

Eventually anyway you will have to organize bookmarks at that large amounts with Opera desktop version. The first thing to do is back up that many bookmarks especially the quick dials which are bound to a specific new installation. With desktop opera syncing, then export all to a .html backup. Then clicking folders selecting sort alphabetically, also doubles after alphabetically sorting show next each other and can easily be sorted out. Make sure before this many bookmarks sorting with desktop version of Opera to have the backup. It's best to do this once a year or so.