(Show Hidden Files in Windows)
go to
C:\Users\User_Name\AppData\Local\Programs\Opera\Version_Number\ (find the last version)
find opera_gx_splash.exe and delete it
Restart PC if it doesn't work immediatly

Another way is this, but I tried and it didn't work

In the address bar digit
opera://flags
it opens a "secret" setting page;
in the search bar (top right) digit intro; it appears Opera One Introduction
click in the drop down menu ed choose "Disabled".
In the page, bottom right, click on "Relaunch"
Restart PC

@steve1376
After every update is acceptable, since I think it reinstalls all the files. But when you use it many times per day is annoying.
I'm not the discoverer of these tricks, I've just found them on the net. The first worked for me; not the second.

(Show Hidden Files in Windows)
go to
C:\Users\User_Name\AppData\Local\Programs\Opera\Version_Number\ (find the last version)
find opera_gx_splash.exe and delete it
Restart PC if it doesn't work immediatly

Another way is this, but I tried and it didn't work

In the address bar digit
opera://flags
it opens a "secret" setting page;
in the search bar (top right) digit intro; it appears Opera One Introduction
click in the drop down menu ed choose "Disabled".
In the page, bottom right, click on "Relaunch"
Restart PC