@Sn00ker Without knowing what distro and desktop environment you're using and whether you're using the Snap or Flatpak or rpm/deb version of Opera, I don't know if I can give specific directions, but:

In a text editor, paste the following and save it as OperaMax.desktop

[Desktop Entry] Name=Opera (Maximized) Exec=opera --start-maximized Type=Application Terminal=false Icon=opera

Then, right-click the file on your desktop, go to properties and make it executable (on permissions tab for example).

Or, find Opera's shortcut in the start menu, copy it to your desktop and then edit it with a text editor to add the --start-maximized part.