[Desktop Entry]
Version=1.0
Name=Opera
GenericName=Web browser
Comment=Fast and secure web browser
TryExec=opera
Exec=opera %U
Terminal=false
Icon=opera
Type=Application
Categories=Network;WebBrowser;
MimeType=text/html;text/xml;application/xhtml_xml;x-scheme-handler/http;x-scheme-handler/https;x-scheme-handler/ftp;application/x-opera-download;
Actions=new-window;new-private-window;

[Desktop Action new-window]
Name=New Window
Exec=opera --new-window
TargetEnvironment=Unity

[Desktop Action new-private-window]
Name=New Private Window
Exec=opera --private
TargetEnvironment=Unity

And save it as opera.desktop under ~/.local/share/applications/ .

What is the second question?

The menu works both with a mouse click and Alt key on the machine with Ubuntu 18.04, but unfortunately it doesn't work at all on the machine with Ubuntu 16.04 (actually there is no O-menu there at all) - i don't know what makes that difference between the two Ubuntu installations (other apps also work differently on the two ubuntu computers) ?

As far as i can remember, the menu had worked before on 16.04 - it has stopped working since some previous version of Opera that i can't remember now.

As for the versions:
@vbnvbn said in No O-menu button at the top-left corner of the browser's window:

The machine:
Current Opera browser version installed: 67.0.3575.137 (x86_64-linux-gnu)
64bit OS: Linux Ubuntu 16.04 64bit - there are many packages installed
Hardware: Intel processor, 4 GB RAM

At the same time, the same version of the browser has no such problem on another machine with Ubuntu 18.04 64bit (also with many packages installed, intel processor, 4GB RAM).

Is there any key combination (such as Alt + another key) for opening the menu? (because i already tried Alt + O and other Alt combinations with no effect)